Presentation, data and programs at: Research designs Hein Stigum Presentation, data and programs at: http://folk.uio.no/heins/ Nov-18 H.S.

Designs Aims Designs Disease occurrence Exposure-Disease association Cross-sectional studies Cohort studies Case-control studies Case-Cohort Nested Case Control Traditional Case Control Aim: find association Designs: unit of analysis, time for finding exposure and disease Nov-18 H.S.

The 2 by 2 table Nov-18 H.S.

True or false? It takes 2 to tango It takes 3 chords to play the blues It takes 4 numbers to be an epidemiologist Nov-18 H.S.

The 2 by 2 table The information is in the contrasts The lowest number sets the precision .01+.01+.1 +.01 =.13 Add 100 to 100 cell: .01+.01+.1 +.005=.125 Add 10 to 10 cell: .01+.01+.05 +.01=.08 To increase power: In a cohort study: oversample unexposed In a case control study: oversample disease The information is in the contrasts Nov-18 H.S.

3 examples What design should we use? Gender and Smoking Do girls smoke more than boys? Exercise and coronary heart disease (CHD) Does exercise reduce the risk? Genes and Diabetes type 1 Does gene-type increase the risk? What design should we use? Nov-18 H.S.

Cross-section Nov-18 H.S.

Time Nov-18 H.S.

Cross-sectional example Pro: fast and inexpensive Con: reversed causality Adolescents age 16-18 95% CI 2.1 3.3 Ex 2: Reversed causality: right after disease, CHD->little exercise, later may have CHD->much exercise OK Nov-18 H.S.

Cohort Nov-18 H.S.

Disease frequency depicted Show: Prevalence Incidence proportion, closed pop, no loss to follow up Incidence rate, takes observation time into account, (could also have loss to follow up) Nov-18 H.S.

Cohort example Pro: reliable Con: costly, time consuming Nov-18 H.S. 95% CI 0.4 0.6 Gene-Diabetes as cohort: Gene freq 5% * dis freq 1%=0.05% Number in exposed, diseased cell=100 000*0.05%=50 100 000 Nov-18 H.S.

Case Control studies Nov-18 H.S.

Cohort vs Case-Control Cases Controls Nov-18 H.S.

Case-Control studies Cohort studies Case-Control studies Measure the exposure experiance of the entire population Case-Control studies Measure the exposure experiance of a sample of the source population of cases (=base) Key assumption Sample controls independent of exposure (same k) Prospective or retrospective Nov-18 H.S.

Case-Cohort . controls Prospective Nov-18 H.S.

. . . . Nested Case-Control case case controls controls risk set Prospective Nov-18 H.S.

Traditional Case-Control . controls Retrospective Nov-18 H.S.

Traditional Case-Control example Pro: small, efficient Con: recall bias 95% CI 1.6 4.7 recall bias Nov-18 H.S.

Calculations Nov-18 H.S.

Data layout OBS, table transposed, b and c different from earlier tables Nov-18 H.S.

Cross-sectional Data Pro: fast and inexpensive Con: reversed causality Reversed causality: diet and stomach cancer Pro: fast and inexpensive Con: reversed causality Nov-18 H.S.

Cohort Data Unbalance in the 2x2 table Exposure is rare Exposure is rare: oversample Disease is rare: increase sample, may be costly, do we need all the persons (or person time) Unbalance in the 2x2 table Exposure is rare Disease is rare Nov-18 H.S.

1) Case Cohort 2) Nested Case Control 3) Traditional Case Control k=sampling fraction Nov-18 H.S.