By Dr Aijaz Ahmed Sohag

Prep by: Abdul Wasay Baloch

Amna Inayat Medical College

Case Control vs Cohort Control

Case Control Cohort Control

Proceeds from effect to cause

Starts with the disease

To know suspected cause occurs more frquently having disease than those without disease

Suitable for study of Rare disease

Relative inexpensive

Proceeds from cause to effect

Starts with exposure

To know whether disease occur more frequently in those exposed to risk factors(a+b) than non exposed (c+d)

Suitable for exposure of Rare response

Comparatively expensive

Case Control Cohort Control

Generally yields only estimate risks (odds ratio)

Time of study relatively short

Population size needed relatively small

Potential bias larger (assessment of exposure)

Generally yeilds relative risk, attributable Risk, besides incidence rate

Relatively large

Comparatively mimic less (assessment of outcomes)

E.g. Smoking and lung cancer.

Fermingham heart study, Oral contraceptive and health

Case Control Cohort Control

RR estimate may increase or decrease as a result of bias

a) Bias due to confounding (may be reduced by Matching)

b) Memory or Recall bias

c) Selection bias

d) Berkesonian bias or Joseph bias

e) Interviewer bias (may be reduced by double blinding)

Examples Adenocarcinoma of Vagina

Oral contraceptive and thromboemolic disease

Thalidomide tragedy

Ethical problems minimal

Incidence can not be measured, and can only estimate the Relative Risk

Incidence Rate Cigrette smoking Developed Lung

cancer Did Not Develop Lung cancer

Total

Yes 70 (a) 6930 (b) 7000 (a+b)

No 3 (C ) 2997(D) 3000 (c+d)

Incidence Rate among Smokers a/(a+b) = 70/7000*1000

= 10 per 1000

Among Non- smokers = c/(c+d)= 3/3000*1000

= 1 per 1000

Estimation of Risk:

Relative risk = incidence of dis among exposed/incidence of dis among non expose= RR = 10/1=10

Attributable risk= incidence of dis among exposed – incidence of disease among non exposed/incidence rate among exposed

=10-1/10 *100 = 90%

RR AR To study etiology (cause) it has an edge

over AR

It measure Strength of association between suspected cause and effect

Larger the RR, stronger the association between Cause and Effect. RR 1 indicates no association

RR has less public health importance as does RR

E.g. if RR is 10, it means smokers are 10 times at greater risk of developing lung cancer than non smokers.

Comparatively AR has less importance studying antilogy of disease

It measures To What extent disease under study Attributed to exposure

e.g. 90 % lung cancer in smokers was due to smoking

AR gives better idea than does RR of the impact of preventive/public health program in reducing problem