What Is Wrong with Obsolescence?

Pedro Alvarez and Isabel EscalonaFaculty of Management, University of Extremadura, Badajoz, Spain. E-mail: palvarez@unex.es

Antonio Pulgarı́n*School of Library & Information Science, University of Extremadura, Badajoz, Spain. E-mail: pulgarin@unex.es

The growth of scientific output in recent years hasmeant that fewer libraries are able to offer the entirerange of journals, with the others being forced to makea selection. The objective of the present work is to de-scribe criteria to regulate the selection of these journalsto provide the researcher with the information that ismost being used in research. One form of quantifyingthis information is by way of the citations that papersreceive over a period of time following their publication.Obsolescence, expressed in terms of an annual agingfactor, does not reflect the real behaviour of most pa-pers. An alternative is the use of “topicality,” consideredas a latent variable, with the Rasch model as the mea-suring instrument. We considered 45 physics journals,and found the results of applying the Rasch model to bemore satisfactory than those obtained with the annualaging factor.

Introduction

Research in all areas of knowledge has undergone aspectacular growth in recent years. Academic journals haveconsequently proliferated to such a degree that libraries areunable to offer the full diversity of publications, and arefinding themselves in a situation where they must selectthose journals that provide the scientist with the informationthat is most being used in research. This poses the problemof determining objective criteria to regulate the form ofselection. One of them would be the current topicality of theresearch theme of the past that has been cited in subsequentyears. This topicality would be reflected in the citations thatappear in the later papers, testimony to the research interestin a certain area of knowledge. If a paper is being cited lessover time, this points to a declining research interest in thework which is consequently losing topicality—it is becom-ing old or obsolescent.

The term “obsolescence” appeared for the first time inthe work of Gross and Gross (1927). The authors analyzed

the references in the 1926 volume of the journalChemicalLiterature, and observed that the number of references fallsto one-half at 15 years.

Subsequent work has approached obsolescence from var-ious points of view. Burton and Kebler (1960) are creditedwith first using the term “half-life” in 1960 in the subject ofscientific literature. They defined the term as “the timeduring which one-half of all the currently active literaturewas published.” These authors found that the half-life de-pended on the subject area.

But it was Brookes (1970a) who first contributed themain advance in knowledge of the obsolescence of thescientific literature, establishing a negative exponentialfunction for obsolescence, and defining such major conceptsas utility, utility factor, and annual aging factor. The methodintroduced to measure obsolescence was graphical, togetherwith an analysis of errors and recommendations for sam-pling.

Following Brookes, obsolescence has been defined byLine and Sandison (1974) as the “decline over time invalidity or utility of information.”

Various workers have applied Brookes’ model, or aderivative, to different bodies of scientific literature. Partic-ularly worthy of note is the work of Griffith et al. (1979),who determine obsolescence for all the scientific literaturecited in the SCI up to that date. Likewise, they determine theaging factor for many journals of the SCI, thereby findingagainst the work of Sandison (1974) who had consideredthat aging did not exist.

Some authors have related obsolescence to the growth ofscientific literature or to the productivity of the journals andthe dispersion of the scientific literature (Brookes, 1970b;De Queiroz & Lancaster, 1979–1981; Line, 1970; Oliver,1971; Ravichandra Rao & Meera, 1992; Sandison, 1975;Stinson & Lancaster, 1987).

Obsolescence may be studied in two distinct ways: syn-chronously or diachronously. In the former case, one fixesthe citing literature and studies the age distribution of thereferences therein; in the latter case, one fixes the cited

* To whom all correspondence should be addressed.

© 2000 John Wiley & Sons, Inc.

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literature and studies the use of this literature following itspublication. Although the two mechanisms are similar, theirare handled differently (Egghe, 1993; Sandison, 1975; Stin-son & Lancaster, 1987).

Recently, Ravichandra Rao and Meera (1992), investi-gating the concrete influence of the growth rate of theliterature on the rate that a piece of that literature becomesobsolete, showed that, in the synchronous case, the fasterthe literature grows, the faster it becomes obsolete.

Lastly, Egghe and Ravichandra Rao (1992) show that theobsolescence (aging) factor, defined by Brookes (1970a) asa time-independent constant, is not constant but a functionof time, because the general citation data are not distributedexponentially as stated by Brookes, but in practice presentan initial increase of citations followed by a “sort of”exponential decay. There is consequently no way to find anaging factor that is time-independent:a is only independentof time in the case of the exponential distribution. Eggheand Rao conclude that only the lognormal distribution canbe used as the underlying distribution.

Description of the Problem

The definition of “obsolescence” according to Line andSandison (1974) could also be applied to aging. Indeed,many authors use these two terms indistinctly.

The manner in which “decline” is specified in the obso-lescence of a paper or journal is by way of how often it issubsequently cited. According to the definition, there wouldonly exist obsolescence if there were a decrease in citationsor references. This poses serious problems when it comes toquantifying obsolescence or aging of a paper over a periodof time, because in most cases there is no decline at thebeginning of the period being considered, but only follow-ing 1, 2, 3, etc., years. One form of comparing journals interms of obsolescence is by means of the aging factor.

Table 1 gives the course of the citations to the journalsPhysics Letters Band Nuovo Cimento Dover the periodfrom 1985 to 1994. The data are the number of times thatthe papers published in 1985 were cited in each of the 10years 1985–1994.

One can see that the difference in citations between any2 years ofPhysics Letters Bimplies a greater loss than thetotal of citations toNuovo Cimento D. The aging factor,however, of Physics Letters Bis 73.7% and ofNuovoCimento D77.7%. Which of the two journals has a smallerdegree of obsolescence in accord with a greater coverage ofsatisfaction on the part of the research community?

Currently, the obsolescence of journals (or of a body ofliterature), as referred to a time period and calculated interms of the aging factor, is expressed by a rate that denotesthe annual loss of current interest. Thus, an aging factor of80% means that with each year that passes, 20% fewerarticles of that set are being cited than in the previous year.This rate, however, is a theoretical rate, which in most casesdoes not coincide with the real loss of citations in anyarbitrarity chosen year.

The question arises therefore of how to replace the agingfactor by some alternative use of the information providedby the citations, a use that would fit reality more closely inbetter reflecting current research interest, and that wouldone to compare and rank the journals. One way is via theterm “topicality.” The journalPhysics Letters Bhas accord-ingly more “topicality” in the 10-year period thanNuovoCimento Dbecause it picks up a greater total of citations.

The pattern of citations over a period of years is charac-terized by a behaviour that is common to all the journals ineach year. There is a rising tendency during the first yearsfollowed by a fall. Each year thus has a distinct particular-ity, and as in the overall period all years should have thesame degree of involvement, they will have to be cataloguedequally. One form of satisfying this goal is take all the datacorresponding to each of the 10 years making up the wholeperiod and transform it to the same number of categoriesexpressed as levels. This procedure will allow us to compareand rank the journals equitably. In a given year, a journalwith many citations will obtain high levels and another withfewer citations will obtain lower levels: the two journalswill be reflecting the same annual idiosyncracy but withdifferent intensities.

When the results, following categorizing, are expressedin terms of raw scores with means or percentages, however,they are descriptive of what is happening at that time, butthey are not measures because they cannot be used to add,subtract, or multiply.

The Rasch Model as Measuring Instrument

The term “topicality,” referring to topicality expressed inthe citations, can be considered to be a latent variable whichis manifest at different levels over the years, reflecting ineach year the same particular characteristic that distin-guishes that year.

Like any other variable, “topicality” is conceived of as adirected line along which the items (years) and journals aresituated (Alvarez & Pulgarı´n, 1996, 1999). One assumes

TABLE 1. Citations to journals from 1985 to 1994.

Journal a 85 86 87 88 89 90 91 92 93 94

Nuovo Cimento D 77.7 10 27 44 41 38 34 27 19 33 26Phys. Lett. B 73.7 1132 5081 5321 4024 3134 2791 2095 1872 1730 1680

a Aging factor in percent.

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there is a single direction that implies “more” of the vari-able. “More” is more distance along the line. It is necessaryto find forms of establishing the locations of the items on thesaid line if we believe that this is one form to help us thinkof “topicality.”

Let the line

b0

ub1

ub2

ub3

ulow“ topicality”

ud1

ud2

ud3

high“ topicality”

represent the latent variable “topicality.” Along it are situ-ated the years, with their specific characteristic,di (i 5 1, 2,3) and journalsbn (n 5 0, 1, 2, 3). Journalb1 exceeds (ismore to the right of) the year represented by the parameterd1. The journal b2 exceeds parametersd1 and d2. Thejournal b3 exceeds all the parametersdi considered. Thejournal with least “topicality” isb0 and that of greatest“topicality” b3. The yeard1 is that of lowest measure andrepresents a citation level reached in journalsb1, b2, andb3,whereas yeard3 is that of highest measure (is most to theright) and represents a rarer citation level which is onlyreached in journalb3.

To illustrate how the theory works, let us restrictXni to bea dichotomous information variable related to journalbn

and yeardi signalling whether that journal scores on thatyear. If the score is 1 (i.e.,Xni 5 1), then the journal hasscored in that year, and so provides some information;otherwiseXni 5 0. A simple extension of dichotomousscoring is to identify one or more intermediate levels ofperformance on an item and to award partial credit forreaching these intermediate levels (Wright & Master, 1982).

A way of relating the position of journal and year to thedichotomous variable is in terms of probabilities

if bn . d i, i.e., if bn 2 d i . 0 then Pr@Xni 5 1# . 0.5

if bn , d i, i.e., if bn 2 d i , 0 then Pr@Xni 5 1# , 0.5

if bn 5 d i, i.e., if bn 2 d i 5 0 then Pr@Xni 5 1# 5 0.5

With the appropriate calculations, one obtains

P$Xni 5 1ubn, d i% 5e~bn2di!

1 1 e~bn2di!

which gives us the probability that journal “n” has beencited in year “i,” referred to the parametersbn anddi (Rasch,1980).

Methodology

The citations we chose for the present work were recov-ered from the Cited Journal Listing, Subject Category“Physics” (referred to the years 1985–1994) which is an

alphabetical list of the journals cited in the ISI database. Themain entry includes the cited journal’s total citations re-ceived for the year, the 10-year distribution of citationsreceived (by year of publication of the cited items), and,under the heading “The Rest,” citations to articles publishedprior to the 10 years listed. For a journal “n,” we consideredthe number of citations received in 1985–1994 to the arti-cles published in 1985 in that journal. The resulting datamatrix consists of 45 rows corresponding to each of thechosen Physics journals, and ten columns corresponding tothe 10 years.

The “topicality” variable takes into account all the cita-tions which each journal has received in the years 1985,1986, 1987, 1988,. . .,1994 of articles published in the year1985. These citations are transformed to a 1–9 scale (Al-varez et al., 1993), expressing nine different levels of cita-tion. One thus builds up the matrix with “n” rows (which arethe different journals) and “i” columns (the year). Theestimation of the parametersbn anddi obtained by applyingthe Rasch model provides a measure of the “topicality” ofthe journals and the years (Wright & Linacre, 1997).

Results and Discussion

The results according to the aging factor ranked in termsof obsolescence, and the results corresponding to the Raschanalysis are given in Table 2.

In the first column appear the journals in order of greaterto lesser measure according to the Rasch model applied tothe term “topicality” considered as a latent variable. Thesecond column gives those same journals in order of lesserto greater obsolescence according to the aging factor. Forthe calculation of the two rankings, we used the same datasource: the citations to the papers published in 1985 thatwere received in the following 10 years.

Although the results correspond to different theoreticalconceptions, they have the same purpose: to provide ajournal selection criterion. If we were to select only the first25 journals out of the 45, the result would depend on whichof the two methods we used. Thus, for example, with theaging factor,Nuovo Cimento Dwould be selected butPhys-ics Letters Bwould not. That is, one of the journals with thegreatest “topicality” value, with a total of 28,860 citations inthe 10 years (1985–1994) would not be selected, whereas onof those least “topicality,” with a total of 299 citations,would. On the contrary, using the Rasch “topicality” mea-sure,Physics Letters Bwould be selected andNuovo Ci-mento Dwould not.

The aging factor method and our method both use thesame input: citations. Neither method takes journal size(number or articles) into account. But the aging factorpresents certain problems as we described perviously. Toovercome these problems, we use same data but a differentmathematical tool. The only difference between the twomethods is therefore the mathematical vehicle, and themarkedly different end results.

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The Rasch model results seem more convincing as areflection of reality than those obtained by the aging factor.

Conclusions

The concept (or term) “topicality,” considered as a latentvariable defined in terms of citations, is a more faithfulreflection of the information that is being used in researchthan the concept of obsolescence defined in terms of anaging factor.

With “topicality,” the behaviour of the citations to paperspublished previously in the journals of an area of knowledge

over a period of time have a common characteristic: at thebeginning of the period, the number of citations rises, to befollowed by a decline. The annual aging factor does notreflect this characteristic.

The criterion obtained with “topicality” leads to a moresatisfactory selection of journals than that provided by theaging factor.

References

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TABLE 2. Order of the journals using the two methods.

By Rasch model By aging factor

1. Phys Rev Lett (24)a 1. Inst Phys Conf Ser2. Phys Lett B (36) 2. Ann I H Poincare-Phy3. Phys Rev A (20) 3. P Roy Soc Edinb A4. Phys Rep (6) 4. Top Appl Phys5. J Phys Soc Jpn (30) 5. Rev Mod Phys6. J Phys A-Math Gen (23) 6. Phys Rep7. Phys Lett A (32) 7. Theor Math Phys18. Jetp Lett1 (26) 8. Am J Phys9. Phys Scripta (17) 9. Rep Prog Phys

10. Rev Mod Phys (5) 10. Chinese Phys11. Prog Theor Phys (27) 11. Nuovo Cimento D12. Ann Phys-New York (12) 12. Ann Phys-New York13. Can J Phys (19) 13. Contemp Phys14. Usp Fiz Nauk1 (33) 14. Ann Phys-Paris15. Z Naturforsch A (22) 15. Gen Relat Gravit16. Rep Prog Phys (9) 16. Riv Nuovo Cimento17. Physica A (38) 17. Phys Scripta18. Theor Math Phys1 (7) 18. Aust J Phys19. Am J Phys (8) 19. Can J Phys20. Classical Quant Grav (29) 20. Phys Rev A21. Acta Phys Pol A (43) 21. Int J Theor Phys22. Aust J Phys (18) 22. Z Naturforsch A23. Gen Relat Gravit (15) 23. J Phys A-Math Gen24. Phys Today (45) 24. Phys Rev Lett25. Czech J Phys (41) 25. Acta Phys Pol B26. Ukr Fiz Zh1 (42) 26. Jetp Lett127. Wave Motion (28) 27. Prog Theor Phys28. Found Phys (37) 28. Wave Motion29. Helv Phys Acta (35) 29. Classical Quant Grav30. Nuovo Cimento D (11) 30. J Phys Soc Jpn31. Int J Theor Phys (21) 31. Nuovo Cimento B32. Nuovo Cimento B (31) 32. Phys Lett A33. Ann Phys-Paris (14) 33. Usp Fiz Nauk134. Acta Phys Pol B (25) 34. Vestn Mosk U Fiz As135. Ann Phys-Leipzig (39) 35. Helv Phys Acta36. P Roy Soc Edinb A (3) 36. Phys Lett B37. Ann I H Poincare-Phy (2) 37. Found Phys38. Contemp Phys (13) 38. Physica A39. Top Appl Phys (4) 39. Ann Phys-Leipzig40. Inst Phys Conf Ser (1) 40. Ptb-Mitt41. Riv Nuovo Cimento (16) 41. Czech J Phys42. Vestn Mosk U Fiz As1 (34) 42. Ukr Fiz Zh143. Fortschr Phys (44) 43. Acta Phys Pol A44. Chinese Phys (10) 44. Fortsch Phys45. Ptb-Mitt (40) 45. Phys Today

a Number in parenthesis is the “aging factor” ranking position.

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