Epidemiology Cheat Sheet

• Incidence= [Number of new cases in a time period/Population] x 100,000

• Prevalence= [Number of existing cases in a time period/population] x 100,000

• Bradford Hill Criteria of causation

1. Strength of Association 
2. Consistency Specificity in the cause 
3. Temporality 
4. Dose-response relationship 
5. Plausibility 
6. Coherence 
7. Experimental evidence 
8. Analogy 

 • Cross-sectional study: assesses both the health status and the exposure levels of individuals within a population at one point in time.

 • Case-control study: a retrospective study that initially identifies two groups of subjects. All individuals in one group have the particular disease or condition under investigation (the cases), whereas everybody in the other group is free from the disease (the controls).

 • Odds ratio (OR): the risk of the odds of developing the disease in the exposed group divided by the odds of developing the disease in the unexposed group.

a= case exposed to risk factor
b= controls exposed to risk factor
c= case not exposed to risk factor
d= controls not exposed to risk factor

OR= (a/c)/(b/d)= ad/bc If exposure if harmful, OR will bew greater than1 If exposure is protective, OR is less than1 If OR=1, no risk can be attributed to the disease

 • Cohort study: Follows a group of subjects forward in time and compares their outcomes after one group is exposed to some known of suspected cause of disease while the other group is not exposed.

• Relative risk: compares the risk of some health-related event occurring in two groups that are included in a prospective study. It is the probability of disease occurring in the exposed group divided by the probability of disease in the unexposed group.

a= those exposed to the risk factor and who have the disease
b= those exposed but who do not have the disease
c= those not exposed to the risk factor and who have the disease
d= those not exposed and who do not have the disease

 RR= [a/(a+b)]/[c/(c+d)] If RR is greater than1, association is positive If RR is less than1, association is negative (or protective) If RR=1, not associated with risk of disease

• Attributable risk: the probability of disease in the exposed group minus the probability of disease in the unexposed group. AR= [a/(a+b)] – [c/(c+d)]

• Absolute risk reduction: the difference in the probability of disease between the treatment and control groups. It is calculated the same as AR, but it tells you the how much of the difference in reduction of disease incidence between the groups is due to the treatment. ARR= [a/(a+b)]- [c/(c+d)]

• Relative risk reduction: the comparative reduction in rates of bad outcomes between the experimental and control groups in an RCT or cohort study. RRR= absolute risk reduction/probability of disease in unexposed group, thus RRR= {a/(a+b)] – [c/(c+d)]}/[c/(c+d)]

 • Number needed to treat: number of patients who would need to be treated in order to prevent one additional bad outcome. NNT= 1/Absolute risk reduction= 1/[a/(a+b)] – (c/(c+d)]