Questions and Best Answers

Courses


Tag

EPIDEMIOLOGY: The study of the distribution & determinations of health related status or events in specified populations & the application of this study to control of health problems.

ESTIMATION OF DISEASE RISK IN CASE OF CASE CONTROL STUDY:

Case control study is called “Retrospective studies”-

                                              Selection of cases & controls

                                                                    ↓

                                                               Matching

                                                                    ↓

                                                          Measurement

                                                                    ↓

                                                 Measurement of exposure

                                                                    ↓

                                                            Analysis- 1. Exposure rates among cases & controls.

                           2. Estimation of disease risk associated with exposure. (Odds ratio)

ODDS RATIO (CROSS PRODUCT RATIO):

It is a measures of the strength of the association between risk factor & outcome.

Closely related to relative risk (Relative risk= Incidence among exposed/ Incidence among non-exposed)

DERRIVATION OS BASED ON THERE ASSUMPTION:

  1. The disease being investigate must be relatively rare.
  2. Cases must be representative of those with the disease.
  3. Controls must be representative of those without disease.

ODDS RATIO = ab/bc

 

                            DISEASE

 

           YES

             NO

Exposed

           a

            b

Non-exposed

           c

            d

 

Example- Odds ratio = (a/b) / (c/d)        

 

 

    Cases (with lung CA)

Controls (without lung CA)

Smokers (<5 cigarettes a day)

33 (a)

55 (b)

Non-smokers

2 (c)

27 (d)

So, Odds ratio = ad/bc = 33*27/55*2 = 8.1

So, smokers showed a risk of having lung cancer 8.1 times that of non-smokers.

→ BIASES IN CASE CONTROL STUDY:

Bias is any systemic error in the determination of the association between the exposure & disease.

  1. Bias due to confounding: it can be removed by matching.
  2. Memory or recall bias: Cases & controls have different recall of past events.
  3. Selection bias: There may be systemic differences is characteristics between cases & controls. It can be controlled by its prevention.
  4. Berkesonian bias: It arises because of different rates of admission to hospitals for people with different disease.
  5. Interviewer’s bias: It occurs when the interviewer’s knows the hypothesis & the cases. It can be eliminated by double blinding.
;