Case- Control Studies

Dr Sanjay JajuHead Specialist

Directorate of Research &

Studies

Directorate General of

Planning

MOH (HQ)

Presentation compiled by

Main Sources of this presentation

Case-Control Studies : Design, Conduct, Analysis by James J. Schlesselman

Essential Epidemiology: Principles and Applications by William Oleckno

Hierarchy of evidence

• Meta analysis / Systemic Reviews• Randomized Controlled double blind trials• Non-randomized trials• Prospective cohort studies• Retrospective cohort studies• Case Control studies• Cross sectional studies• Case series• Case reports

Important discoveries in case-control studies

• Cigarette smoking and lung cancer• Post-menopausal estrogens and

endometrial cancer• Relation of Toxic Shock Syndrome to the use of

tampons• Association between use of diethylstilbesterol

during the 1st trimester of pregnancy and the development, fifteen to twenty years later of vaginal cancer in the daughters born of these pregnancies

• Aspirin and Reyes syndrome• AIDS and sexual practices• Vaccine effectiveness• Diet and cancer• Reduced risk of fractures with the use of

estrogens• Maternal smoking and congenital

malformations• Herpes Simplex virus and Bells palsy• Low dose radiation and leukemia

etcetera

A type of observational analytical epidemiological

investigation in which the subjects are selected on the

basis of whether they do (cases) or do not (controls)

have a particular disease under study.

The groups are compared with respect to the

proportion having a history of an exposure or

characteristic of interest.

Definition

Case-control study design can be Unmatched or Matched

Design of a Case-Control study

Population

Sample **

Outcome absent

(Controls)

Outcome present

(Cases)

Exposure status? Exposure status?

** Frequently in case control studies cases and controls are selected separately

Time 1

Time 2

Disease onset

Initial symptoms

Clinical Diagnosis

Study exposure is unlikely to be altered at this stage because of the disease onset.

Therefore, exposure assessment during this period is more likely to reflect the exposure that preceded disease onset.

Study exposure is more likely to be altered at this stage because of the disease symptoms.

Therefore, exposure assessment during this period is less likely to reflect the exposure that preceded disease onset.

Diet & stomach cancer

Determining Exposure

Establishing proper temporal sequence between exposure and outcome

To determine causal association exposure must precede outcome

Study must use incident (new) cases and then assess prior exposure

NOT SURE WHETHER

exposure preceded outcome?

outcome preceded exposure?

exposure and outcome occurred simultaneously?

exposure is related to

? Disease development ? Disease prognosis

If one uses prevalent cases

OR

Determining incident (new) cases is not always easy

It is not clear when the new cases began !!!!!!

For practical purposes-Incident cases are defined as newly diagnosed cases (date is taken as proxy)

Acute cases: Date of outcome approximates the date of diagnosise.g: automobile related injuries, food poisoning etc

Chronic cases: Date of diagnosis may be years after disease has developede.g: multiple sclerosis, emphysema, depression etc

Case selection

Criteria (reliable and widely acceptable)

Definition of a case must be clear and precise with

strict inclusion / exclusion criteria

Evidence from clinical examination & appropriate diagnostic tests

to prevent dilution with non-cases and hence spurious associations

eg: different diseases having similar signs and symptoms but

unrelated causes.

General Population based

less common due to time and

expenses, but feasible in case of

population based registries.

(assures representativeness and

external validity)

Hospital based

certain hospitals, clinics,

nursing homes, rehabilitation

centers

Include all Incident Cases in a defined population over specific time period

Specific Population basedpre-paid health plan, schools,

places of employment, military service etc

affects external validity

but not internal validity

Retrospective case selection All leukemia cases diagnosed for the first time between 2000 and 2012 at a particular medical center.Advantage • increases power of study if time period is further increased .Disadvantages• Requires accurate database• Detailed information on exposure• Diagnostic methods may have changed over time• Disease classifications may have changed over time

Prospective case selection All newly diagnosed leukemia cases from Jan 2014 onwards till Dec 2015at a particular medical center.Disadvantages• Expenses• May not accumulate the required number of cases to assure power of the

study• Diagnostic methods may change over time• Disease classifications may change over time

Most difficult and most controversial aspect of study design

The control group provides a basis for comparison by representing what is normal or expected.

The control series is intended to provide an estimate of the exposure rate that would be expected to occur in the cases if there was no association between the study disease and exposure.

Selection of Controls

Selection of Controls (contd)

• Should come from the same source population and time period that has given rise to cases.

• Must be free of the disease under study

• Comparable to the cases with respect to confounding factors like age, sex, racial/ethnic background, socioeconomic level, occupational status and certain lifestyle behaviors (this is achieved by Restriction, Matching or Statistical adjustment during analysis)

Selection of Controls (contd)

• Comparable to the cases regarding to the possibility of having past exposure during the time period of risk. This avoids selection bias.

• Restrictions/exclusions for cases must also be applied to controls.

Confounding

Is the distortion in the degree of association between an exposure and an outcome due to mixing of effects between an exposure and an incidental factor (confounder)

For confounding to occur:• The exposure and incidental factor must be associated• The incidental factor must be a risk factor for the outcome• The incidental factor cannot be an intermediate step between the exposure and outcome• The incidental factor must be present to a greater or lesser degree in the study group versus the comparison group.

e.g.: Positive association has been shown between cigarette smoking and motor vehicle injuries. What is the confounder?

Exposure Outcome

Confounder

Methods of Selecting ControlsPopulation controls

A) Probability sample of General Population from which cases come. Problems- Selection bias due to low rates of participationMeasurement bias possibly due to poor recall

B) Probability sample of Specific Population from which cases come eg: school rosters, selective service lists, insurance company lists, neighbors of cases, friends, schoolmates, siblings, fellow-workers.

Problems-Overcomes the above two bias. But, may not represent the true range of exposure that exists in the source population due to too much similarity with cases as regards exposure status.

Hospital controlsPersons seeking medical care at the same institution as cases for condition not related to cases’ disease.

• Convenient• Better comparability (except for referral/ tertiary care hospitals)• Better participation • Less recall bias

ProblemsMay not represent the exposure distribution in the source population as they represent ill people who may be more likely to have an unfavorable risk profile than healthy people in the source population.

CaveatHospital controls from diagnostic categories known to be associated with the study exposure must be excluded. eg: In testing hypothesis that cigarette smoking causes lung cancer, cases with chr. bronchitis, emphysema must not be included as controls. Therefore choose controls from a variety of other diagnosis.

Define Exposure· Yes / No· Intensity· Length of exposure eg: cigarette smoking

Ascertainment of Exposure· Personal interviews

· Existing records· Physical measurements and lab tests

Exposure

Accurately determining exposure status

is essential to avoid measurement bias

Interviewer bias: unconscious tendency to search more thoroughly for exposure among cases than controls.Same assessment for cases and controls is a must.Verification from other sources: lab records, official records, information from spouses, relatives, colleagues etc

Recall bias: cases often remember past exposures better than controls due to natural tendency to know why the disease occurred.

Sample size issues

Using multiple controls per case will lower the sample size requirement for the case group but increase the total sample size needed.

Up to 4 controls per case assures adequate study power

The results of the case control study will be inconclusive if there is insufficient study power ( which is related to sample size)

Strengths• Relatively quick and

inexpensive to conduct• Appropriate for studying

rare outcomes• Require moderate number

of subjects• Multiple potential risk

factors can be examined in the same study

• If properly executed, then cause-effect relationship can be established

Weaknesses• Incidence rates in exposed and

unexposed subjects ordinarily cannot be determined

• Not appropriate for studying rare exposures

• Information on prior exposure or potential confounders may not be readily available, accurate or of the same quality between cases and controls (eg due to differential recall)

• It may be difficult to identify comparable case and control groups, and thus there is greater potential for selection bias.

Advantages of Case-Control Study

Analysis of unmatched case control studies

Controls are NOT pair matched with cases during selection process

a b

c dExposure status

Exposed

Unexposed

Outcome status

Cases Controls

a + b

c + d

a + c b + d N = a+b+c+d

Odds that cases were exposed a/(a+c) c/(a+c)

Odds that controls were exposed b/(b+d) d/(b+d)

Odds ratio = ratio of 2 odds

Ratio of odds in favor of exposure among cases to odds in favor of exposure among controls.

a/(a+c)c/(a+c)b/(b+d)d/(b+d

____________

a/cb/d

= = ad / bc

Significance of OR is given by Chi-square Χ2 = n(ad –bc)2 (a+b)(b+c)(a+c)(b+d)

The 95% confidence interval for the OR must be stated.

Odds Ratio

Case-control Study - Example:

To study the association between smoking and lung cancer

• People with lung cancer are enrolled to form the case group, and people without lung cancer are identified as controls.

• Researchers then look back in time to ascertain each person’s exposure status (smoking history)

Variables Used in Study of Smoking and Lung Cancer

Subject Selection Control selection• Age matched• Healthy individuals• Patients hospitalized for other cancers• Patients hospitalized for other diseases• Deaths from other causes than cancer• Sampling of general population

• Males and/or females • Occupational groups • Hospitalized cases • Autopsy series • Total lung cancer deaths in an area• National sampling lung cancer deaths

Methods of Interviewing • Mailed questionnaires• Personal interviews subjects/relatives• Personal interviews controls

Other Variables Concurrently Studied• Geographic distribution• Occupation• Marital status• Coffee and alcohol consumption• Other nutritional factors• Parity• War gas exposure• Other pathologic conditions• Hereditary factors• Air pollution• Previous respiratory conditions

Tobacco-Use History • Type of smoking• Amount and type• Duration • Inhalation practices

Association between smoking and lung cancer

Cases Controls(with lung cancer) (without lung cancer)

Smokers 33 (a) 55 (b)Non-smokers 2 (c) 27 (d)

Odds Ratio = ad / bc

= 33*27 /55*2 = 8.1

Interpretation:Lung cancer is 8 times more likely in smokers compared to non-smokers

Matched case control studies

Advantages• Comparability on selected

variables• Control selection easy –

avoids random sampling• Useful in small studies where

obtaining cases and controls that are similar on potentially confounding factors may be difficult

• Assures adequate number of cases and controls for statistical comparison

Disadvantages• Difficult to find controls if

many variables are being matched

• Cannot examine the effects of matched variables

• Overmatching due to nearly similar groups

• Loss of data as pair has to be eliminated if one subject is non responsive

a* b*

c* d*

Control

Exposed Unexposed

Exposed

UnexposedCase

a+b

c+d

n* *b+da+c

Odds ratio = b / c

Significance of OR is given by Chi-square Χ2 = (b – c)2 (b+c)

Analysis of matched case control studies

* represents a pair

** n= ½ the total number of cases and controls in the study

Thank

you