ANALYTICAL STUDY DESIGNS

CASE CONTROL STUDY

LEARNING OBJECTIVES

To develop an understanding of……….

➔ What case-control studies are??

➔ The value of such studies

➔ The basic methodology

➔ Pros and Cons of such studies

Lesson plan:Target group: Third year BDS students

LO Content Method Time Media

1 Study designs introduction Explanation 5 min Slide 5– 6

2 Analytical study design Explanation 5 min Slide 7 - 10

3 Case control study design Explanation 10 min Slide 11 - 18

4 Matching Explanation 5 min Slide 19 -21

5 Odds ratio Explanation 10 min Slide 22 - 24

6 Bias in case control Explanation 10 min Slide 25 - 27

7 Pro and cons of case control Explanation 5 min Slide 27 - 28

CONTENTS STUDY DESIGNS – RECAP

ANALYTICAL STUDY DESIGNS

INTRODUCTION TO CASE CONTROL STUDY

DESIGN OF A CASE CONTROL STUDY

ELEMENTS OF A CASE CONTROL STUDY

MATCHING

ODDS RATIO – CALCULATION AND INTERPRETATION

PROS AND CONS OF CASE CONTROL STUDY

SUMMARY

STUDY DESIGNS - RECAP

Epidemiological Study CycleDESCRIPTIVE STUDY • Ca Lung increasing mostly smokers

• Death rates higher in populations with higher per

capita cigarette consumption

Ochsner, 1939

CASE CONTROL STUDY • Ca Lung patients and non patients

Clarifies if it was smokers who contributed to high Ca Lung

Doll, 1947-52

COHORT STUDY • Follows a cohort of smokers and non smokers without Ca Lung

• Smokers develop Ca Lung more frequentlyINTERVENTIONAL TRIAL (RCT) • Proves hypothesis conclusively

• Gives inputs regarding other factors, control measures.

Hypothesis: Smoking causes

Ca Lung

Hill, 1951-61

Analytical studiesHistory of medicine has always been fascinated in discovering the causes of the disease and the ways in which these could be modified.

CAUSES OF DISEASE

EVENT, CONDITION, CHARACTERISTIC OR COMBINATION OF FACTORS

DISEASE

EXPOSURE

SEVERAL OBSERVATIONS

HAVE TO BE MADE

ANALYTICAL EPIDEMIOLOGY

Analytical studies Investigator does not assign the exposure

Makes careful measurement of patterns of exposure

and disease in populations

Comparison group

Make inferences about exposure and disease

RESEARCH DESIGNS IN ANALYTICAL STUDIES

COHORT STUDY

CROSS SECTIONA

LSTUDY

CASE CONTROL

STUDY

CASE CONTROL COHORT STUDIES

RETROSPECTIVE

Ca Lung patients and non patients

Clarifies if it was smokers who contributed to high Ca Lung

PROSPECTIVE

Follows a cohort of smokers and non smokers without Ca Lung

Smokers develop Ca Lung more frequently

Case-Control StudiesThe observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison/ reference) group of persons without the disease. (Dictionary of Epidemiology: 3rd ed; John M Last. 2000)

Case-Control StudiesA case control study involves two populations – cases and controls and has three distinct features : Both exposure and outcome have occurred before

the start of the study. The study proceeds backwards from effect to cause. It uses a control or comparison group to support or

refute an inference.(Park’s Textbook of Preventive and Social Medicine – 20th ed; K. Park. 2009)

Design of case‐control studyObjective: Test association between cigarette smoking and lung cancer(Doll and Hill, 1952)

EXPOSED

NON EXPOSED

EXPOSED

NON EXPOSED

CASES(with lung cancer)

CONTROLNon – cancer patients

Exposureodds

Exposureodds

ODDS

RATIO

EXPOSED

SMOKER

NON EXPOSED

NON SMOKER

OUTCOMETIME

EXPOSURE

ELEMENTS OF A CASE CONTROL

STUDY

1. SELECTION OF CASES

2. SELECTION OF

CONTROLS

3. INFORMATION ON

EXPOSURE

4. ANALYSIS

Selection of cases• All people in source population who develop the disease of interest Sample of cases Independent of the exposure under study• Clear definition of outcome studied• Prevalent vs. incident cases Prevalent cases may be related more to survival with disease than to development of disease.

Sources of cases

• Hospital/clinic based cases Easier to find May represent severe cases

• Population based (cancer registry) - not biased by factors drawing a patient to a particular hospital

Selection of controls • Represent the distribution of exposure in the source population of cases -Selected from the same source population that gives rise to the cases

• Selected independently of their exposure status

SELECTION OF CONTROLS

Population based• Sampling of the general population

Health care facility based• Patients with other diseases

Case‐based• Friends, Neighbourhood

MATCHINGDefined as “ the process by which we select controls in such a way that they are similar to cases with regard to certain pertinent selected variable which are known to influence the outcome of disease and which if not adequately for comparability could distort or confound the result ”

TYPES OF MATCHINGType 1 Group Matching: assigning cases to subcategories based on their characteristics like age occupation, etc. and then establishing appropriate controls.

Type 2Pair matching: It is finding a control for particular case as closely resembling as possible except for disease under study.

Selecting good data on exposure1.Objectively

• Reproducibility of exposure measurement

2. Accurately• Information reflecting as closely as possible the effect of exposure

3.Precisely• Quality management in exposure measurement

Presentation of the data of a case‐control study in a 2 x 2 table

ODDS RATIO

INTERPRETING ODDS RATIO • OR = 1 -Odds of exposure among cases and controls are same -Exposure is not associated with disease

• OR > 1 -Odds of exposure among cases are higher than controls -Exposure is positively associated with disease

• OR < 1 - Odds of exposure among cases are lower than controls - Exposure is negatively associated with disease

Bias in case control studiesBIAS – is a systematic error in design, conduct or

analysis of a study which leads us to an erroneous

conclusion.1.Bias in selection of cases - selection bias or diagnostic bias

2.Bias in investigating controls. - recall bias, the controls are less likely to recall exposure variables than the cases.

-The interview/tests/investigation etc may lack depth in controls whereas the cases are thoroughly worked up

3. CONFOUNDING BIAS

(distortion of study effect with another effect because of variables EXTRANEOUS to the exposure affecting the prediction of the disease)

When the disease has multiple risk factors which are related to each other

SOLUTION – MATCHING BETWEEN CASES AND CONTROLS

4. Problems due to over matching : - This is where a potential confounder ( religion in substance abuse) is matched among cases and controls. The study thus loses the power of proving an obvious association.

5.Bias in analysis - the presence of a confounder is mostly identified at the time of analysis. - It is due to non- uniform distribution of confounders.

Solution – Stratification ( limit the size of study and no of confounding factors)

STRENGHTS

Good for examining rare outcomes or outcomes

with long latency Relatively quick to conduct, inexpensive Requires comparatively few subjects Multiple exposures or risk factors can be examined.

WEAKNESSES

Susceptible to recall biasSelection of an appropriate comparison group may be difficult

Rates of disease in exposed and unexposed individuals cannot be determined

SUMMARYSTUDY DESIGNS

• DESIGN• ELEMENTS• MATCHING• ODDS RATIO• BIAS• STRENGTHS & WEAKNESSES

OBSERVATIONAL EXPERIMENTAL

ANALYTICAL DESCRIPTIVE

CASE CONTROL COHORT CROSS SECTIONAL

REFERENCES

1) Soben Peter. Essentials of Public Health Dentistry. 5th ed. New Delhi: Arya Publising House; 2013.

2)Park, Park’s Textbook of Preventive &Social Medicine, 22nd Edition, Jabalpur: Banarsidas Bhanot,2013.

THANK

YOU!