SJS SDI_141 Design of Statistical Investigations Stephen Senn 14
Case Control Studies Slide 2 SJS SDI_142 Case-Control Study
Definition The observational epidemiologic study of persons with
the disease (or other outcome variable) of interest and a suitable
control (comparison, reference) group of person with the disease.
The relationship of the attribute to the disease is examined by
comparing the diseased or nondiseased group with regard to how
frequently the disease is present, or if quantitative, the levels
of the attribute in each group. In short the past history of
exposure to a suspected risk factor is compared between cases and
controls, persons who resemble the cases in such respects as age
and sex but do not have the disease or condition of interest. Last,
J.M. A Dictionary of Epidemiology Slide 3 SJS SDI_143 Schematic
Representation of Cohort Study Each point represents a member of
the cohort of 10,000 persons Slide 4 SJS SDI_144 200 cases and 200
controls are sampled from diseased and healthy persons respectively
Slide 5 SJS SDI_145 The number of cases and controls is a foregone
conclusion. Exposure becomes the random variable and is studied as
a function of status Note that axes have been exchanged to reflect
this Slide 6 SJS SDI_146 Smoking and Lung-Cancer Obs_7 Famous study
of Hill and Doll Sampled 1357 cases of lung cancer from four
hospitals in the United Kingdom Sampled 1357 hospital-based
controls Compared the two groups as regards smoking history Slide 7
SJS SDI_147 Doll and Hill Data Obs_7 Slide 8 SJS SDI_148 In General
Slide 9 SJS SDI_149 A Model for Case-Control Studies Number exposed
Number unexposed Probability case if exposed Probability case if
unexposed Probability recorded if case Probability recorded if
control Slide 10 SJS SDI_1410 Expectations etc. Slide 11 SJS
SDI_1411 Notes Thus the odds-ratio can be estimated even though n
E, n U, and are unknown. However, although the assumption that and
are equal is not needed, an assumption that they do not vary with
exposure is needed. Slide 12 SJS SDI_1412 Sources for Controls (
Rothman ) Population using population register Neighbourhood For
example one or two control from neighbourhood of case Not suitable
for environmental exposure Random digit dialing Hospitals or
clinics Slide 13 SJS SDI_1413 Cohort and Case Control Studies
Cohort Case Control Complete population Can calculate incidence
rates Usually expensive Convenient for studying many diseases Can
be prospective or retrospective Sampled population Can calculate
ratios only Usually less expensive Convenient for studying many
exposures Can be prospective or retrospective Rothman p 91 Slide 14
SJS SDI_1414 The Delta Method Slide 15 SJS SDI_1415 Variance of a
Logit Slide 16 SJS SDI_1416 Variance of the log-odds ratio The
log-odds ratio is the difference between two logits. Since these
are independent, the variance of their difference is the sum of
their variances. Thus, in terms of our previous table, we have Note
the implications of the variance formula. The variance cannot be
reduced beyond the reciprocal of the entry in a given cell by
increasing the frequencies of the other cells. Slide 17 SJS
SDI_1417 S-Plus Analysis Obs_7 #Doll and Hill
options(contrasts=c("contr.treatment", "contr.poly")) #set contrast
options #To analyse the famous case-control study Outcome
