Leicester Warwick Medical School Health and Disease in Populations Revision Paul Burton

What you don’t need to know!!!!! Detailed knowledge of how to fill in a death certificate Detailed knowledge of census questions Detailed knowledge of exactly what is in/not in specific information sources Knowledge of web site addresses Wording of Helsinki declaration, Hippocratic Oath Mathematical formulae for error factors

Cohort Studies Exposed Unexposed Time Count events and pyrs

A worked example 1,000 children followed from birth to age 5 yrs 300 at least one parent smoked in the home 700 neither parent smoked in the home p-y SMOKE = 300  5 pyr = 1,500 p-y p-y NON-SMOKE = 700  5 pyr = 3,500 p-y

A worked example Smoke exposed, 75 diagnosed asthma Smoke unexposed, 105 diagnosed asthma IR SMOKERS = 75/1,500 = 50 per 1,000 p-y IR NON-SMOKERS = 105/3,500 = 30 per 1,000 p-y

A worked example IRR = e.f. = e.f. = % CI: 1.667÷1.35 to  1.35 i.e to 2.25

Case-control Studies Case Non-Case (Control) Exposed? Time

Analysis 95%CI: OR  e.f., OR  e.f.

Creutzfeld Jacob Disease (CJD) and occupation Odds ratio = (9×104)/(3×13) = 24 95% CI: 24÷4.29, 24×4.29 = (5.59, 103.0)

How many controls? Unlike an IRR, the precision of an OR is affected by the number of healthy people (x and z): So, it is worth increasing the number of controls - up to a point (typically up to 4-6 times as many controls as there are cases)

Multiple levels of exposure

Retrospective v prospective? Confusing terminology: two different issues (1) Does the analysis look forwards or backwards? (2) Are the data collected as and when they occur ( i.e. prospectively) or from historical review - questionnaire, case-notes or other health records – ( i.e. retrospectively). Cohort analysis always looks forwards in time: Given exposure status at baseline, how many events occurred over time in how many person years and what is the incidence rate ratio? Simple case-control analysis is usually expressed as being backwards in time: Given case-control status now, what is the ratio of the odds of exposure at baseline?

Retrospective v prospective? Confusing terminology: two different issues (1) Does the analysis look forwards or backwards? (2) Are the data collected as and when they occur ( i.e. prospectively) or from historical review - questionnaire, case-notes or other health records – ( i.e. retrospectively). Conventional cohort study: prospective Historical cohort study: retrospective Conventional case-control study: retrospective

Comparison of cohort and case-control studies

Statistical inference on a rate ratio Population 1: d 1 cases in P 1 person years Population 2: d 2 cases in P 2 person years Rate ratio = d 1 /P 1  d 2 /P 2

An example 80 deaths in 8,000 pyrs (male) 50 deaths in 10,000 pyrs (female) Rate M = 10 per 1,000 p-y; Rate F = 5 per 1,000 p-y Observed rate ratio (M/F) = % CI: [2÷1.43 to 2×1.43] = [1.40 to 2.86] Best guess for true rate ratio=2.0, and 95% certain that true rate ratio lies between 1.40 and This range does not include 1.00 so able to reject hypothesis of equality (p<0.05)

Statistical inference on an SMR Observe O deaths Expect E deaths (based on age-specific rates in the standard population and age-specific population sizes in the test population) SMR = (O/E)  100

For example On basis of age specific rates in standard population expect 50 deaths in test population. Observe 60. (O=60, E=50) SMR = (60/50)×100 = % CI for SMR = 120 ÷/× 1.29 = 93 to 155. CI includes 100 so data consistent with equality of death rate in test and standard populations (p>0.05). But also consistent with e.g. a 50% excess so certainly doesn’t prove equality.