Comparison of internal reliability and validity of the McGill Pain Questionnaire and the Short Pain Inventory

ORIGINAL ARTICLE.........................................................................................................................

Comparison of internal reliabilityand validity of the McGill PainQuestionnaire and the Short PainInventoryShaun G. Kilminster1,2 and Graham P. Mould1

1Guildford Clinical Pharmacology Unit and 2Department of Neurology, Royal Surrey County Hospital, Guildford, Surrey GU2 7XX, UK

Correspondence to: Dr Shaun G. Kilminster, Guildford Clinical Pharmacology Unit, Royal Surrey County Hospital, Guildford, SurreyGU2 7XX, UK. E-mail: [email protected]

Received 21 December 2001; accepted 29 January 2002

SummaryWe compared the psychometric properties of the McGill Pain Questionnaire (MPQ) withthe 17-item Short Pain Inventory# (SPI) in 60 outpatients with osteoarthritic knee pain.Split-half reliability, Guttman split-half reliability, Cronbach alpha and the correlationbetween the first and second half of the test were higher in the SPI total pain disturbancethan any of the summary or subscales of the MPQ: 0.94, 0.94. 0.88 and 0.90, versus 0.71,0.70, 0.69 and 0.55 for the MPQ sensory present pain intensity, 0.59, 0.57, 0.65 and 0.43 forthe affective and 0.62, 0.55, 0.49 and 0.45 for the evaluative MPQ scale, respectively. Theparameters for the SPI total mood disturbance were superior to all MPQ-derived scales.Dividing into high and low ‘pain experienced right now’ identified screening samples. TheSPI ‘pain right now’ was more discriminating than the comparative MPQ item on both theSPI and MPQ. Additionally, none of the summary scales of the MPQ could show significantinternal discrimination whereas the SPI did achieve this.There were 23 significant correlations with the SPI severity compared with 15 with the

MPQ. SPI sadness, anxiety, anger, total mood disturbance and total pain disturbance weresignificantly correlated with the MPQ severity rating. The MPQ variables fared less wellthan the SPI in the degree of association between various pain parameters and physicalseverity.Factor analyses revealed that the SPI accounts for the majority of the variance (50%)

compared with 17% for Factor 2. This second factor is best indexed by the McGill ‘painnow’ and also with the SPI ‘pain severity now’ item. Since the SPI indexes the severity asaccurately (0.79) as the McGill, the only difference between the two is the sensory MPQvariance. However, since the SPI (Factor 1) also indexes some of the common variance ofthe MPQ sensory variable, the SPI also gains in this respect. If it were the case that theoutcome of an analgesic clinical trial was the sensory aspects of the pain (cutting, throbbing,rasping) then the McGill should be the obvious outcome measure. For the patient, the mostimportant feature of pain surely must be the physical severity and the unpleasantness of theexperience. The MPQ is a rather long procedure and the evaluative scale is of dubious value.The majority of the pain variance is captured by the SPI and, secondly, by the sensory aspectof the MPQ. The SPI measures the emotional aspects of pain well and the McGill assessesthe physical or sensory aspects of pain better than any other available. Both have their placeaccording to one’s research interests and the clinical relevance. For example, if aninvestigation involves opiates like morphine, the SPI may be the better placed instrument,since the induced euphoria may present as a patient who can still feel the pain but is no

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longer bothered by it. Algesimetry requires measuring both the physical and emotionalsensations of the patient.

Keywords: McGill Pain Questionnaire, Short Pain Inventory, psychometrics, comparison,osteoarthritis

Introduction

The McGill Pain Questionnaire (MPQ) is probably the mostwell-known pain rating scale to date. Some consider it to be thegold standard in pain-summated rating scales [1]. The main partof the MPQ has 78 items divided into 20 subclasses, which arethen grouped into sensory, affective, evaluative (cognitive) andmiscellaneous. The MPQ often requires completion with thehelp of a research assistant, whereas the Short Pain Inventory#(SPI) is completely self-administered [2–5]. The SPI wasdeveloped to measure those aspects of pain the McGill doesnot do so well at, namely the emotional aspects of pain. TheMPQ indexes the physical or sensory aspects of pain well andthe SPI indexes the emotional aspects of pain to a higher level ofaccuracy and sensitivity.

The SPI is a 17-item self-rating questionnaire dedicated tomeasuring the immediate physical pain and emotional con-sequences of pain. Patients rate each item on a five-point Likertscale: ‘not at all’, ‘a little’, ‘moderately’, ‘very much so’ and‘extremely’. Five subscales measure sedation, social interaction,anxiety, sadness and anger and have been fine-tuned bymaximizing items that are specific to pain. The SPI takes aboutone minute to complete. A scale of total mood disturbance canbe summated from the individual items. The SPI measures thesubtle mood changes that specifically co-vary with mild tosevere pain. Factor analytical and discriminant functionanalyses were used in the development of SPI [6,7]. Sedationitems, such as ‘I feel drowsy’, were omitted because they did notdiscriminate between individuals in pain from those without.Items like ‘distressed’ were very discriminating but impaired theoverall internal reliability as indexed by the Cronbach alpha. Inshort, we have found that around 70% of the variance ofphysical pain is common to the overall mood of the patient [2,8].

In the development of the MPQ, a numerical value is given toeach of the words or, more simply, a count of the number ofwords chosen. From an item pool of 102 pain descriptors, theywere grouped according to the similarity of meaning. Originally20 university graduates then ranked the ordering of the words.A second ranking involved 140 psychology students of whom90% were men who were predominantly not in pain. The sampleincluded 20 physicians and 20 patients. Use of this young andrelatively well-educated group may explain why words likelancinating, lacerating, taut and wrenching may cause confusionwith the average patient on trials and in the clinic.

The scoring system assumes that the descriptors within asubclass are equidistant on an ordinal scale, which is not thecase [9]. The MPQ does not give equal consideration to the threemain dimensions. The number of words, upon which a numberranking system is based, varies (two in subclass 11, and six insubclass one). Their descriptor word lists involves 10 subclasses(1–10) for the sensory pain rating index (PRI). Yet only fivesubclasses (11–15) are used for the affective PRI. The evaluativePRI is from only one subclass. With items like ‘miserable’,‘annoying’ and ‘troublesome’ included in the miscellaneoussubclass, one may question why they were not included inmeasuring mood change. Likewise, ‘killing’ and ‘blinding’ items

do not seem to be likely candidates for measuring mood. Part 4deals with ‘how strong is your pain right now’. The Likertresponse gives ‘mild’, ‘discomforting’, ‘distressing’, ‘horrible’and ‘excruciating’. There is no response allowed for ‘no pain’.No pain assessment should omit the measurement of the absenceof pain. One final problem with the MPQ in use is that it iscumbersome and time consuming. A short form of the MPQ [10]has been developed with only 15 items ranked with a five-pointLikert scale, which is not used here. The short form is basedupon the original. We will produce empirical evidence to showweaknesses in the traditional long MPQ format. The MPQ takeson average five minutes to complete and sometimes needs aresearch assistant to give help, whereas the SPI takes an average70 s without help. Part 2 is the main part of the MPQquestionnaire involving the 20 subclass rankings. This alonetakes about 150 s to complete.

Pain is an important cause of attendance at medical clinics andmay be a consequence of the treatments provided. Historicallypain has been measured by subjective rather than objectivemethods since the latter have not been easy to use and areinsensitive and non-specific [11–14]. The anaesthetistmay look forrespiratory and cardiovascular responses to various surgicalinsults during an operation to infer the patient’s subjective pain.Induced pain may be measured simply by the time to keep a handsubmerged in the cold pressor tests [12,15], or by the more exoticcognitive after-effects of a painful stimulus [12] to the use of theevoked potential [11]. More recently, laser-induced pain tech-niques have successfully measured changes of only 2% insubjective pain with only 12 volunteers [14]. None of theseresearch methods has translated into better pain measures in theclinic. Despite many laboratory advances, the mainstay ofsubjective assessment in clinical pain is still rating of a single itemby either the visual analogue scale (VAS) method or by the Likertscale method [16,17]. The VAS typically involves a 10 cm line andis polarized with no pain or worst pain ever. This is considered bymost pain authorities to be a gold standard [18–23].

One may ask how could the single visual analogue method beimproved? One method is the Likert scale [17]. This involvesbreaking up the visual analogue into four or more scaleddescriptor components such as ‘no pain’, ‘a little pain’,‘moderate pain’ and ‘severe pain’. We find the elderly patientoften prefers the Likert, finding it easier to understand, and theclinician does not need the use of a ruler. Additionally, the VASmethod often gives a non-normal distribution that cannot beanalysed by the more powerful parametric methods used inclinical trials [6,16]. Summated rating scales, namely the addingup of several ranked items, have the superior psychometricproperties of normalized distributions and better point estimatesof the average (mean) [7,24]. This is because although one itemmay be an overestimate of what is being measured, another itemmay be an underestimate; the summation of several rating itemstends to cancel out the errors of measurement [7]. Perhaps themost compelling reason for not using a single VAS to measurepain is that this must be an oversimplification of the total painexperience [25]. Pain is undoubtedly a multidimensionalexperience and measurement ought to be consistent with this.

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The pain item that is scaled often implicitly refers to physicalpain yet it is quite possible for a patient on a high dose ofmorphine to respond, ‘yes I feel the pain but it does not botherme’. Indeed, euphoria may ensue with morphine producing adissociation between normal pain and feeling ‘not pleased’ withlife [26]. The physical characteristics of the pain, be it throbbing,stabbing, cutting, etc., can be quite varied and the dedicatedpain questionnaire such as the MPQ is pre-eminent in this field[27].

The affective counterpart of pain is well known to all of uswho have had the misfortune to hit our finger with a hammer.Sudden bursts of pain can cause anger and anxiety and someresearchers have used this phenomenon to review pain-inducedmood with a dedicated test such as the 20-item State-TraitAnxiety Inventory (STAI) [28] or the 65-item Profile of MoodStates (POMS) [29]. The STAI is an excellent measure of anxietywith high validity and reliability in both patients and normals.While the STAI indexes only anxiety, the POMS reviews sixmood states and takes about four minutes to complete. Theaffectual component of pain is becoming increasingly identifiedas an essential assessment of pain [2,4,8].

Though mood disturbance is obviously found in individualswithout physical pain, we have shown empirically that negativehedonic tone is increased in pain in a variety of painfulconditions [4]. It is also likely that adverse mood states might‘amplify’ any limited physical pain. In this sense, mood statesoperate as both causes and consequences of pain. Concentrationdifficulties have also been shown after pain stressors [12]. TheSPI includes two items, concentration and politeness, as thesehave been shown to discriminate those in pain from thosewithout. Item selection was generated from a sample of 41000and a sample of 4300 patients in pain and from subjects notexperiencing pain in the final 17-item version. Recently, the SPIhas been shown to be sensitive enough to differentiate thedifferences in various medical and dental patient groups [4].

We set out to compare and contrast the MPQ and SPIadministered to 60 osteoarthritic knee pain patients at ascreening clinic. We compared the discriminability, internalreliability, and structure and redundancy of the MPQ and SPI,as indices of pain.

Methods

The design was a within-group comparative study in two hospitaloutpatient osteoarthritic knee pain clinics, comparing the internalreliability and validity of the MPQ and SPI. The study wasapproved by the South West Surrey Local Research EthicsCommittee and the East Sussex, Brighton and Hove HealthAuthority Local Research Ethics Committee as part of an analgesicclinical trial. The questionnaires were administered by rheumatol-ogy clinic staff at Eastbourne General Hospital. A psychologist(S.G.K.) administered the questionnaires at the Royal SurreyCounty Hospital (Clinical Pharmacology Unit). The SPI andMPQwere self completed in all cases but assistance was sometimes givenwith the MPQ because of its relative complexity.

Explanation on how to complete the questionnaires was keptto a minimum and all spoiled forms were entered into theanalysis on an ‘intention to treat’ basis. This was done so as toreduce administrator bias and to evaluate the SPI as it would beused realistically in clinical screening settings. Data were takenfrom the first completion of each test and it is obviously the casethat repeated presentation warranted in clinical trials would

improve on the reliability and validity. However, this was a‘warts and all’ study. Analysis was by discriminant functionanalysis, simple t-tests, test–retest Pearson product momentcorrelation coefficients, Cronbach alpha and split-half reliabil-ity. The latter two are standard tests of reliability. Statisticalanalysis was carried out on a Power Macintosh G4/400/200machine with Statistica V4. All data points were doublevalidated against source forms.

Statistical methods

The internal reliability was computed with the Cronbach alphafor the 17 items in the SPI and the 20 subclass values plus (painright now) on the MPQ. The Guttman split-half reliability wasalso computed on odd versus even numbered sequential items.For the internal validity, high and low scores were median spliton both the MPQ and SPI ‘pain right now’ parameters. Thushigh and low pain groups were divided by both pain severitymeasures as screening samples. Summary and derivative MPQand SPI measures were then used as dependent variables insimple t-tests. Discriminant function analysis is the same assimple t-tests here where there is only one independent and onedependent variable. Factor analyses were carried out to examinethe structure and redundancy of the two tests.

Results

Demographic detail

At entry the mean age was 63 years (SD=9.4), the mean weightwas 83.9 kg (SD=17.2) and the mean body mass index (BMI)was 30.1 kg/m2. There were 37 females and 27 males. A total of43 patients (67%) were taking at least one maintenanceanalgesic at study entry. Extreme pain (4) was reported by 9%of patients at screening, very much pain (3) by 41% of patients,moderate pain (2) by 45% and a little pain (1) by 5% ofpatients. Four patients withdrew from the study.

Internal reliability of SPI and MPQ

The psychometric properties of the MPQ summary and subscaleswere compared with the SPI. SPITPD refers to the complete 17-item scale measuring the total pain disturbance. The SPITMD

refers to the total mood disturbance excluding physical painseverity and social interaction. The MPQ includes all the itemsfrom part 1 of the MPQ. The MPQ1–27 includes all the rankedvariables available on the MPQ form. The MPQ1–21 refers to allof part 1, but with the addition of the rating for ‘pain right now’.The PRI variable was the total of all sensory, affective andevaluative scores. Table 1 shows the SPI parameters to besuperior to the MPQ on all reliability parameters.

Internal validity of SPI and MPQ

Patients were divided into those high and those low in ‘pain rightnow’. Amedian split sample was divided by theMPQ and the SPIscores on the ‘pain right now’ scales. Each of the subscales andsummary scores was tested for the ability to discriminate betweenhigh and low pain. For an equitable comparison, this was doneboth ways; Table 2 shows the MPQ individual items tested with at-test when divided into high (HI) and low (LO) pain severity onthe MPQ ‘pain right now’. Table 3 shows the MPQ summaryscales tested with a t-test when divided into HI and LO pain

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Table 1. Internal reliability of Short Pain Inventory (SPI) and McGill PainQuestionnaire (MPQ)

SPITPD SPITMD MPQ1–27 MPQ1–21

Split-half reliability 0.941 0.917 0.790 0.824Guttman split-half 0.940 0.917 0.790 0.824Cronbach alpha 0.881 0.878 0.759 0.812Correlation for first and second half 0.900 0.846 0.653 0.701

MPQ MPQ MPQ MPQ1–20

sensory affective evaluative PRI total

Split-half reliability 0.711 0.598 0.618 0.847Guttman split-half 0.700 0.575 0.554 0.846Cronbach alpha 0.694 0.653 0.492 0.823Correlation for first and second half 0.551 0.427 0.448 0.735

TPD, total pain disturbance; TMD, total mood disturbance. Significantresults are in bold.

Table 2. McGill Pain Questionnaire (MPQ) part 1 screened by high and lowMPQ ‘pain right now’

MPQ t-test LO HI LO HI LO HIquestion p n n mean mean SD SD

Q1 0.599 24 27 2.12 2.444 2.03 2.259Q2 0.690 24 27 1.17 1.333 1.43 1.519Q3 0.087 24 27 1.50 2.370 1.77 1.779Q4 0.263 24 27 0.46 0.630 0.51 0.565Q5 0.415 24 27 2.33 1.926 1.81 1.730Q6 0.959 24 27 0.83 0.852 1.27 1.292Q7 0.525 24 27 0.71 0.889 0.81 1.155Q8 0.991 24 27 0.71 0.704 1.43 1.463Q9 0.236 24 27 2.92 3.407 1.47 1.448Q10 0.236 24 27 1.12 0.741 1.15 1.130Q11 0.220 24 27 0.88 1.148 0.80 0.770Q12 0.676 24 27 0.21 0.259 0.41 0.447Q13 0.969 24 27 0.42 0.407 0.83 0.844Q14 0.470 24 27 0.67 0.889 1.09 1.086Q15 0.368 24 27 0.29 0.444 0.46 0.698Q16 0.120 24 27 2.25 2.852 1.15 1.512Q17 0.190 24 27 1.29 1.889 1.49 1.695Q18 0.900 24 27 1.17 1.222 1.58 1.552Q19 0.344 24 27 0.25 0.111 0.68 0.320Q20 0.392 24 27 1.29 1.593 1.04 1.394

HI and LO, see text for explanation.

Table 3. McGill Pain Questionnaire (MPQ) part 2 screened by high and lowMPQ ‘pain right now’

t-test HI LO HI LO HI LOMPQ variable p n n mean mean SD SD

PAINCHAN 0.056 23 23 1.435 1.174 0.507 0.388PAINOW 0.000 24 27 1.542 3.074 0.509 0.781PAINWORS 0.005 24 27 3.833 4.519 0.963 0.700PAINLEAS 0.001 24 27 1.292 2.037 0.464 0.980WORTOOTH 0.992 23 26 4.304 4.308 1.222 1.158WORHEAD 0.466 23 27 3.652 3.889 1.071 1.188WORSTOMA 0.059 23 27 3.304 4.037 1.329 1.344PRI sensory 0.501 24 27 13.875 15.296 7.380 7.549PRI affective 0.318 24 27 2.458 3.148 2.670 2.214PRI evaluative 0.172 24 27 6.250 7.666 3.054 4.104PRI total 0.298 24 27 22.583 26.111 11.714 12.179

HI and LO, see text for explanation; PAINCHAN, pain change; PAINOW,pain right now; PAINWORS, pain at worst; PAINLEAS, pain when least;PRI, pain rating intensity; PRI sensory, Q1–Q10 inclusive; PRI affective,Q11–Q15 inclusive; PRI evaluative, Q16–Q20 inclusive; WORHEAD, worstheadache; WORSTOMA, worst stomach pain; WORTOOTH, worsttoothache. Significant results are in bold.

Table 4. Short Pain Inventory (SPI) (subscales) screened by McGill PainQuestionnaire (MPQ) high versus low ‘pain right now’

LO HI LO HI LO HISPI t-test p n n mean mean SD SD

SOCINT 0.807 22 26 1.773 1.654 1.771 1.573Sedation 0.530 24 24 5.625 5.208 2.081 2.467Sadness 0.325 24 25 4.292 5.200 2.926 3.440Anxiety 0.766 24 23 7.208 7.565 3.718 4.430Anger 0.094 24 24 0.458 1.250 0.977 2.048TMD 0.341 24 21 18.958 21.619 7.832 10.651TPD 0.301 22 21 20.000 23.190 8.106 11.643

HI and LO, see text for explanation; SOCINT, social interaction; TMD, totalmood disturbance; TPD, total pain disturbance.

Table 5. Short Pain Inventory (SPI) (items) screened by McGill PainQuestionnaire (MPQ) high versus low ‘pain right now’

SPI LO HI LO HI LO HIquestion t-test p n n mean mean SD SD

SEVERITY 0.000 24 26 1.375 2.308 0.647 0.838Polite 0.901 23 26 0.957 0.923 0.976 0.891Q3 0.721 23 26 0.826 0.731 0.937 0.919Q4 0.591 24 26 0.708 0.885 1.233 1.071Q5 0.740 24 25 1.750 1.640 1.189 1.114Q6 0.355 24 24 0.917 1.208 0.881 1.250Q7 0.739 24 24 1.958 2.083 1.197 1.381Q8 0.140 24 24 0.333 0.792 0.702 1.318Q9 0.818 24 25 1.958 2.040 1.160 1.306Q10 0.577 24 25 1.667 1.840 1.049 1.106Q11 0.337 24 25 1.875 1.560 1.116 1.158Q12 0.272 24 24 1.417 1.083 0.929 1.139Q13 0.211 24 25 0.375 0.720 0.770 1.100Q14 0.495 24 25 0.500 0.680 0.780 1.030Q15 0.954 24 25 2.458 2.440 0.932 1.261Q16 0.473 24 25 1.542 1.760 0.884 1.200Q17 0.151 24 25 0.125 0.440 0.338 1.003

HI and LO, see text for explanation; SEVERITY, nociceptive pain severity.

Table 6. McGill Pain Questionnaire (MPQ) (part 1) screened by Short PainInventory (SPI) high versus low pain severity ‘right now’

MPQ LO HI LO HI LO HIvariable t-test p n n mean mean SD SD

Q1 0.837 23 36 2.174 2.056 2.167 2.124Q2 0.029 23 36 0.739 1.583 1.287 1.481Q3 0.071 23 36 1.391 2.250 1.672 1.795Q4 0.018 23 36 0.304 0.639 0.470 0.543Q5 0.617 23 36 1.870 2.111 1.766 1.817Q6 0.952 23 36 0.826 0.806 1.302 1.238Q7 0.014 23 36 0.391 1.028 0.722 1.055Q8 0.189 23 36 0.435 0.972 1.308 1.630Q9 0.964 23 36 3.261 3.278 1.287 1.485Q10 0.111 23 36 0.609 1.111 0.839 1.326Q11 0.006 23 36 0.652 1.222 0.647 0.797Q12 0.049 23 36 0.087 0.306 0.288 0.467Q13 0.102 23 36 0.174 0.528 0.491 0.941Q14 0.067 23 36 0.435 0.944 0.945 1.068Q15 0.025 23 36 0.130 0.472 0.344 0.654Q16 0.009 23 36 1.870 2.861 1.180 1.477Q17 0.049 23 36 1.000 1.833 1.477 1.595Q18 0.221 23 36 0.870 1.389 1.424 1.661Q19 0.789 23 36 0.174 0.139 0.576 0.424Q20 0.231 23 36 1.217 1.611 1.126 1.271

HI and LO, see text for explanation. Significant results are in bold.



severity on the MPQ ‘pain right now’. Table 4 shows the SPIsummary items tested with a t-test when divided into HI and LOpain severity on the MPQ ‘pain right now’. Using the MPQ ‘painright now’ as a category discriminator proved elusive with the SPIdata. Table 5 shows the MPQ individual items tested with a t-testwhen divided into HI and LO pain severity on the MPQ ‘painright now’. Table 6 shows the MPQ individual items tested with at-test when divided into HI and LO pain severity on the SPI ‘painright now’.

The total number of significant MPQ items from part one,when screened with the high/low SPI, was seven. Table 7 showsthat the total number of significant MPQ summary parameterswhen screened with the high/low SPI pain severity was six.

Table 8 shows that the total number of significant summarySPI parameters, when screened with the high/low SPI painseverity, was three significant and two marginally significant.Table 9 shows that a total of four SPI individual items weresignificant when screened with high/low SPI severity. Summarydata of all these screening data is presented in Table 10.

Cross MPQ and SPI validity

Within this sample, if both SPI and MPQ are measuring aspectsof pain then we can simply identify this by examining the

correlation coefficients across tests. The correlation betweenindividual items on the MPQ and SPI were compared with thesimple rating of severity for each scale and are presented inTable 11.

There were 23 significant correlations with the SPI severityscale compared with 15 for the MPQ. The SPI sadness, anxiety,anger, total mood disturbance and total pain disturbanceparameters were significantly correlated with MPQ severityrating. Table 12 shows the correlation matrix for the summaryMPQ and SPI data. Table 13 shows the factor structure for theMPQ and SPI.

Discussion

It is best to discriminate between internal and external reliabilityand validity. The internal validity refers to the discrimination ofthe degree of the condition (here pain severity) and the reliabilityto the consistency within the questionnaire (Cronbach alpha andothers). External validation refers to the ability of thequestionnaire to measure and discriminate with other painfulconditions and to correlate with other questionnaires measuringthe same construct. External reliability is commonly measuredby test–retest correlations under the same controlled conditions.For example, we administered the SPI to 12 healthy volunteers,under standard laboratory conditions, and the test–retestreliability correlations ranged from 0.975 to no lower than 0.9over 4 h for all SPI parameters. A previous survey of pain intwo medical and dental clinics using the SPI discriminated

Table 7. McGill Pain Questionnaire (MPQ) summary screened by highversus low Short Pain Inventory (SPI)

HI LO HI LO HI LOMPQ t-test p n n mean mean SD SD

PAINCHAN 0.12722 28 20 1.1785 1.4000 0.39002 0.5982PAINOW 0.00005 32 18 2.7187 1.6111 0.92402 0.6978PAINWORS 0.00164 32 18 4.4687 3.6666 0.76133 0.9074PAINLEAS 0.01500 32 18 1.8437 1.2777 0.88388 0.4608WORTOOTH 0.40273 30 18 4.2000 4.5000 1.2703 1.043WORHEAD 0.27583 32 17 3.9062 3.5294 1.1175 1.178WORSTOMA 0.10461 32 17 3.9062 3.2352 1.3995 1.251PRI sensory 0.05840 36 23 15.833 12.000 7.9264 6.578PRI affective 0.00180 36 23 3.4722 1.4782 2.4317 2.019PRI evaluative 0.00682 36 23 7.8333 5.1304 3.7071 3.441PRI total 0.00888 36 23 27.138 18.608 12.449 10.67

HI and LO, see text for explanation; PAINCHAN, pain change; PAINOW,pain right now; PAINWORS, pain at worst; PAINLEAS, pain when least;PRI, pain rating intensity; PRI sensory, Q1–Q10 inclusive; PRI affective,Q11–Q15 inclusive; PRI evaluative, Q16–Q20 inclusive; WORHEAD, worstheadache; WORSTOMA, worst stomach pain; WORTOOTH, worsttoothache. Significant results are in bold.

Table 8. Short Pain Inventory (SPI) summary screened by high versus lowSPI

LO HI LO HI LO HIMPQ t-test p n n mean mean SD SD

SEVERITY 0.000 25 36 0.880 2.444 0.332 0.607Notcounted

SOCINT 0.905 25 34 1.800 1.853 1.780 1.598Sedation 0.069 24 34 4.583 5.676 2.041 2.319Sadness 0.403 25 35 4.120 4.829 3.018 3.339Anxiety 0.083 25 33 5.920 7.818 3.741 4.268Anger 0.017 25 34 0.280 1.235 0.614 1.860TMD 0.007 24 31 15.250 22.097 7.514 9.850TPD 0.018 24 29 17.000 23.586 7.973 11.053

HI and LO, see text for explanation; SEVERITY, nociceptive pain severity;SOCINT, social interaction; TMD, total mood disturbance; TPD, total paindisturbance. Significant results are in bold.

Table 9. Short Pain Inventory (SPI) items screened by high versus low SPI

LO HI LO HI LO HIMPQ t-test p n n mean mean SD SD

Polite Q2 0.608 25 35 0.920 1.057 1.077 0.968Q3 0.739 25 35 0.880 0.800 0.971 0.868Q4 0.223 25 36 0.560 0.917 1.003 1.180Q5 0.015 24 35 1.167 1.886 1.090 1.078Q6 0.055 25 34 0.680 1.206 0.802 1.149Q7 0.038 25 34 1.520 2.206 1.005 1.366Q8 0.014 25 34 0.160 0.794 0.374 1.200Q9 0.486 25 35 1.840 2.057 1.068 1.259Q10 0.935 25 35 1.680 1.657 1.108 1.027Q11 0.343 25 35 1.400 1.686 1.080 1.183Q12 0.617 25 34 1.160 1.294 0.987 1.031Q13 0.560 25 35 0.480 0.629 0.918 1.003Q14 0.451 25 35 0.480 0.657 0.770 0.968Q15 0.528 25 35 2.280 2.457 0.891 1.172Q16 0.395 25 35 1.400 1.629 0.816 1.140Q17 0.114 25 35 0.120 0.429 0.332 0.917

HI and LO, see text for explanation. Significant results are in bold.

Table 10. Number of significant differences between high and low painseverity ‘right now’

Screened by MPQ Screened by SPI

MPQ (Q1–Q20) 0 10SPI (Q2–Q17) 0 4

MPQ SPI

MPQ (10 summary) 4 6SPI (7 summary) 0 5



highly significant differences between patient groups [4]. TheCronbach alpha was 0.93 and the split-half reliability was 0.94.

The present data are consistent with those publishedpreviously [2]. In an analgesic (non-steroidal anti-inflammatorydrug) clinical trial of 40 osteoarthritic patients the internalreliability of the SPI was reassuringly excellent at 0.94 and agreeswell with the present data. With the same patients, the seven-daytest–retest reliability was: 0.71 for severity, 0.76 for socialinteraction, 0.72 for sedation, 0.66 for sadness, 0.57 for anger,0.77 for anxiety, 0.75 for total mood disturbance. Amixed sampleof n=268 also showed an internal reliability of 0.93.

Table 11. Correlation coefficients for McGill Pain Questionnaire (MPQ) andShort Pain Inventory (SPI) versus pain severity on each scale (internalvalidity)

Item: MPQ pain now SPI pain now

MPQ Q1 NS NSMPQ Q2 NS 0.317MPQ Q3 NS 0.258MPQ Q4 NS NSMPQ Q5 NS NSMPQ Q6 NS NSMPQ Q7 NS 0.387MPQ Q8 NS NSMPQ Q9 NS NSMPQ Q10 NS NSMPQ Q11 NS 0.314MPQ Q12 NS NSMPQ Q13 NS 0.388MPQ Q14 NS NSMPQ Q15 0.305 0.431MPQ Q16 0.498 0.518MPQ Q17 0.293 0.282MPQ Q18 NS NSMPQ Q19 NS NSMPQ Q20 0.348 0.298MPQ PAINCHAN 70.365 70.287MPQ PAINOW 1.000 0.678MPQ PAINWORS 0.560 0.420MPQ PAINLEAS 0.678 0.534MPQ WORTOOTH NS NSMPQ WORHEAD NS NSMPQ WORSTOMA NS NSMPQ PRI sensory NS 0.299MPQ PRI affective 0.292 0.477MPQ PRI evaluative 0.349 0.445MPQ PRI total NS 0.416SPI SEVERITY 0.678 1.00SPI SOCINT NS NSSPI sedation NS NSSPI sadness 349 0.269SPI anxiety NS 0.331SPI anger 0.3169 0.385SPI TMD 0.429 0.507SPI TPD 0.405 0.482Mean 0.132 0.209

PAINCHAN, pain change; PAINOW, pain right now; PAINWORS, pain atworst; PAINLEAS, pain when least; PRI, pain rating intensity; SEVERITY,nociceptive pain severity; WORHEAD, worst headache; WORSTOMA,worst stomach pain; WORTOOTH, worst toothache; TMD, total mooddisturbance; TPD, total pain disturbance.

Table 12. Short Pain Inventory (SPI) and McGill Pain Questionnaire (MPQ) correlation matrix

SPI

MPQ Severity SOCINT Sedation Sadness Anxiety Anger TMD TPD

PAINCHAN 70.379 NS NS 70.320 NS 70.300 70.263 NSPAINOW 0.544 NS 0.280 0.409 0.265 0.264 0.395 0.378PAINWORS 0.427 NS NS 0.354 NS NS 0.325 0.328PAINLEAS 0.612 NS 0.500 0.624 0.483 0.277 0.612 0.582WORTOOTH NS NS NS NS NS NS NS NSWORHEAD NS 70.255 NS NS NS NS NS NSWORSTOMA NS NS NS NS NS NS NS NSPRI sensory NS NS NS NS 0.260 NS NS NSPRI affective 0.279 0.233 0.287 0.292 0.401 NS 0.374 0.378PRI evaluative 0.258 NS NS NS NS NS NS NSPRI total NS NS NS 0.253 0.306 NS 0.276 0.268

PAINCHAN, pain change; PAINOW, pain right now; PAINWORS, pain at worst; PAINLEAS, pain when least; PRI, pain rating intensity; PRI sensory, Q1–Q10 inclusive; PRI affective, Q11–Q15 inclusive; PRI evaluative, Q16–Q20 inclusive; TMD, total mood disturbance; TPD, total pain disturbance;WORHEAD, worst headache; WORSTOMA, worst stomach pain; WORTOOTH, worst toothache.

Table 13. Factor analysis of McGill Pain Questionnaire (MPQ) and ShortPain Inventory (SPI)

Factor loadings (varimax rotated solution)

Factor 1 Factor 2 Factor 3 Factor 4 Factor 5

MPQ PAINCHAN 0.04 70.63 70.18 0.51 70.10MPQ PAINOW 0.24 0.84 70.11 0.09 70.06MPQ PAINWORS 0.15 0.75 70.16 0.00 70.18MPQ PAINLEAS 0.38 0.74 70.13 0.03 70.04MPQ WORTOOTH 70.07 0.03 0.05 70.10 70.70MPQ WORTHEAD 0.06 0.11 0.14 0.81 70.04MPQ WORSTOMA 70.15 0.21 70.07 0.42 70.66MPQ PRI sensory 0.21 70.01 70.88 70.12 70.15MPQ PRI affective 0.31 0.28 70.72 70.22 0.25MPQ PRI evaluative 0.05 0.27 70.84 0.19 0.10MPQ PRI total 0.21 0.13 70.95 70.07 70.02SPI SEVERITY 0.28 0.70 70.31 0.09 0.02SPI SOCINT 0.45 70.21 70.30 70.26 0.05SPI sedation 0.66 0.10 70.17 0.09 0.38SPI sadness 0.90 0.16 70.01 0.05 70.02SPI anxiety 0.90 0.05 70.12 70.01 0.08SPI anger 0.52 0.22 70.02 70.49 70.24SPI TMD 0.93 0.23 70.25 70.03 0.09SPI TPD 0.94 0.18 70.29 70.05 0.07

Variance 25.3% 16.5% 17.8% 8.2% 6.8%

PAINCHAN, pain change; PAINOW, pain right now; PAINWORS, pain atworst; PAINLEAS, pain when least; PRI, pain rating intensity; PRI sensory,Q1–Q10 inclusive; PRI affective, Q11–Q15 inclusive; PRI evaluative, Q16–Q20 inclusive; SEVERITY, nociceptive pain severity; SOCINT, socialinteraction; TMD, total mood disturbance; TPD, total pain disturbance;WORHEAD, worst headache; WORSTOMA, worst stomach pain;WORTOOTH, worst toothache.



Table 1 shows the SPI parameters to be superior on allreliability parameters to the MPQ. We can compute theadditional number of items (classes) needed for the MPQ toreach the Cronbach alpha of the SPI, which is 11. For anequivalent internal reliability the MPQ1–20 part 2 needs 31subclasses. With the SPI, item one relates to pain severity rightnow and is included above. However, with the MPQ this is notincluded (MPQ1–20) and to be a fairer comparison it should beincluded in both (MPQ1–21). For an equivalent internalreliability (Cronbach alpha) of the SPI, the MPQ1–21 needsanother 15 classes, each with 2–6 words, namely 36 subclasses.The total number of word items went from 78 to 136 words.This part of the test took about 260 s or just over 4 min. Sincethe SPI takes 70 s to complete all its sections, the time needed tocomplete the MPQ would need nearly four times that needed forthe SPI.

The internal validity of the MPQ individual items from part 1(Table 2) revealed low power to discriminate HI from LOosteoarthritic knee pain felt ‘right now’. None of the individualitems could resolve a significant difference on its own. We thencompared the ability of the summary MPQ scales to bediscriminated by HI versus LO pain severity in Table 3.

The internal validity of the MPQ data revealed some powerto discriminate HI from LO osteoarthritic knee pain felt ‘rightnow’ on pain ‘at its least’, ‘worst pain’ and ‘pain change’. The‘pain right now’ does not count because that was the variablethat was dichotomized to provide the median split. The MPQitems that were discriminating were really variations on a themeof physical severity right now. None of the summary PRIvariables was sensitive enough to distinguish high from low painlevels when using the MPQ to stratify physical pain into HI andLO severity. Table 4 shows the SPI summary items tested with at-test when divided into HI and LO pain severity on the MPQ‘pain right now’. Using the MPQ ‘pain now’ as a categorydiscriminator proved elusive with the SPI data.

Since the SPI subscales did not resolve any differencesdifferentiated on the MPQ ‘pain right now’, we conclude theMPQ ‘right now’ item itself is a blunted measure. This isprobably because the question arises very late on in the MPQand, by the time patients have completed it, some of the obviousdiscrimination is lost. For this reason, the SPI deliberately hasthis question as the first item to be ranked. To prove this pointfurther, we also used the SPI pain severity item to show greaterdiscriminability on both SPI and MPQ measures (Tables 6 and7).

The total number of significant MPQ items from part 1,when screened with the high/low SPI sample, was seven. Thisshows the SPI physical severity scale is a more valid measurethan the MPQ because none was discriminated with the MPQHI/LO stratification. Table 7 shows that the total number ofsignificant MPQ summary parameters when screened with theHI/LO SPI pain severity was six. This shows the SPI severityscale is a more valid measure than the MPQ. Where and how thequestion is asked makes a difference to the scale (a subtlety thatis often lost on novices to questionnaire development). Table 8shows that the total number of significant summary SPIparameters when screened with the HI/LO SPI pain severitywas three significant and two marginally significant. Withsummary pain indices, the SPI can reveal internal validitywhereas the MPQ could not.

It can be seen from Table 10 that the SPI ‘pain right now’ ismore discriminating than the comparative MPQ item on boththe SPI and MPQ. This is the case for both individual items and

summary items. The total number of significant or marginallysignificant tests was 25 for the SPI compared with four for theMPQ. If we screen by the MPQ and ask how many MPQindividual items are discriminated then the answer is none in thissample. The summary MPQ data are better in that they canresolve four out of 10 of summary MPQ items, which is to beexpected since they were pain severity items. Remarkably, noneof these was an MPQ summary scale.

If we screen by the SPI and ask how many SPI individualitems are discriminated then the answer is four out of 16.Interestingly, if we use the SPI ‘pain right now’ scale, we obtainten of the individual MPQ items, which compares with zerowithin its own scale. Similarly, if we use the SPI ‘pain right now’scales, we obtain six of the MPQ items, which compares withfour within its own scale.

The obvious feature of the inter-correlation Table 12 is ageneral absence of relationship of the MPQ worst toothache,headache and stomach ache variables. They do not correlatewith pain severity or anything else but social interaction.Contrasting this is the result of the MPQ ‘pain right now’ thatcorrelates with the SPI pain severity (at this moment) and all ofthe SPI scales except social interaction. The MPQ ‘pain rightnow’ seems to correlate more strongly with SPI variables thanthe MPQ ‘pain at worst’. The highest correlation of the MPQand SPI was between pain at least with the SPI total mooddisturbance. The MPQ PRI affective index correlated with all ofthe SPI scales, excepting anger. This shows the expectedconstruct validation in that we have a greater consistency ofrelationship between the SPI and MPQ for the affective domainof pain. Of note was our previous comparison study of differentpain patients that noted that osteoarthritic knee pain patientsare generally low in anger [4]. The lack of relationship betweenanger and MPQ is probably a floor effect in this patient group.In general, the evaluative, sensory and historical worst, stomach,tooth and headache do not correlate well with the SPI. Onewonders how much the SPI and MPQ overlap in measuring thesame thing. To answer this, factor analysis was carried out toexamine the structure and redundancy of the two pain scales.

From Table 13 it can be seen that Factor 1 accounts for thelargest proportion of the variance (25%) and is best indexed bythe SPI total pain or total mood disturbance (TPD or TMD).‘Pain right now’, ‘pain at least’ and ‘pain at worst’ from theMPQ loads well (0.7) with pain severity from the SPI. The MPQpresent pain rating (PRI) intensity total is indexed from thesensory, affective and evaluative. Interestingly, the PRI affectivedoes not load highly with subcomponents of the SPI mood ortotal mood disturbance. The MPQ does not measure pain-related mood disturbance well. Factor 4 seems best indexed byworst headache (0.8) Factor 5 is best indexed by worsttoothache (0.7) and stomach ache (0.6). One wonders franklywhat use these variables are in a pain clinic context or researchcontext. Factors 4 and 5 we shall call ‘historical factors’. Factor3, PRI sensory, affective and evaluative do not correlate wellwith the MPQ’s own index of ‘pain right now’ (0.11). Curiouslyit correlates better with the pain severity (right now) on the SPI(0.31). By taking the SPI mood scales and the SPI severity ratingfrom Factor 2, we gain most of the variance in this group. Thebest index from the MPQ appears to be the PRI sensory indexand the ‘pain now’ index (Factor 2 of 0.8). This reduced subsetwas further factor analysed and is shown in Table 14.

The SPI accounts for the majority of the variance (50%compared with 17%) of the Factor 2, which is best indexed bythe McGill ‘pain now’ and the SPI ‘severity’. Since the SPI



indexes the severity as well (0.79) as the McGill, the onlydifference between the two is the sensory MPQ variance.However, since the SPI (Factor 1) also indexes some of thecommon variance of the MPQ PRI sensory variable, the SPIgains in this respect. If it were the case that the outcome of ananalgesic clinical trial was the sensory aspects of the pain(cutting, throbbing, rasping) then the McGill should be theobvious outcome measure. However, it is difficult to imagineany such study being done or recalling one that has been done.

From the patient’s point of view, the most important featureof pain must surely be the physical severity and the unpleasant-ness of the experience. In those patients with opiate-inducedeuphoria, dissociation between the physical sensation and theconcomitant mood disturbance may be seen. In such cases, thepatient feels the pain, but it does not bother them. Thecorrelation or factor loading of severity 0.37 with Factor 1 isconsistent with the relationship of mood disturbance with painseverity in mild pain. In osteoarthritis, the pain severity isgenerally mild and the correlation between mood disturbanceincreases to around 0.7 in conditions with greater pain such uschronic daily headache or dental pain. A simplified summary ofthe psychometric results is given in table 15.

Conclusions

The SPI and MPQ share some common pain variance. The SPIhas better discriminability, and superior reliability than any of

the subscales or summary scales provided by the MPQ. The bestsubscale of the MPQ is the PRI sensory scale. Factor analyticalscores show the majority of the variance is dominated by the SPI.Discriminant analyses buttress these findings. The MPQ is toolong and cumbersome, taking about 5–10 times longer tocomplete and is computationally complex. The MPQ oftenrequires assistance since subjects find it difficult to complete. TheSPI takes about 70 s to complete and is computationally simple.It is perhaps no surprise to the reader; there was a need to developan instrument that indexes both the emotional aspects the patientbrings to the physical pain experience and the emotionalconsequences that the physical pain produces. Where there is aneed to measure the physical sensory aspects of pain, the MPQ isgood and bettered by no other scale at present. The MPQ haslower discriminative power compared with the SPI. The MPQfailed to show any of the subscales to discriminate sampledichotomization and hence has lower internal validity bycomparison with the SPI summary scales. Additionally, factoranalytical studies show that the SPI captures the majority of thevariance over the MPQ. Where the clinical researcher isinterested in the physical or sensory aspects of pain, the MPQassesses this well. Where the clinical researcher wishes to measurethe emotional aspects of pain, the SPI is the better instrument.

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Table 14. Second factor analysis of McGill Pain Questionnaire (MPQ) andShort Pain Inventory (SPI) (quartimax rotated solution)

Variable Factor 1 Factor 2

MPQ PAINOW 0.289 0.836MPQ PRI sensory 0.438 0.125SPI SEVERITY 0.376 0.793SPI SOCINT 0.578 70.356SPI sedation 0.754 70.159SPI sadness 0.841 0.119SPI anxiety 0.893 0.028SPI anger 0.502 0.347SPI TMD 0.969 0.207SPI TPD 0.991 0.138Variance 0.5004 0.1695

PAINOW, pain right now; PRI, pain rating intensity; SEVERITY, nociceptivepain severity; SOCINT, social interaction; TMD, total mood disturbance;TPD, total pain disturbance.

Table 15. Summary of McGill Pain Questionnaire (MPQ) and Short PainInventory (SPI): psychometrics

Variable

MPQ Sensory (physical) internal reliability goodMPQ Affective internal reliability poorMPQ Miscellaneous, + worst stomach ache, worst toothache poorMPQ Pain right now and state dependency poorMPQ Internal discrimination of factors poorMPQ Self-administration poorMPQ Percentage of pain variance poorSPI Sensory (physical) internal reliability NASPI Affective internal reliability goodSPI Pain right now and state dependency goodSPI Internal discrimination of factors goodSPI Self-administration goodSPI Percentage of pain variance: good



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Comparison of internal reliability and validity of the McGill Pain Questionnaire and the Short Pain Inventory