Formulae Flashcards
Calculate RR
AR in exposed group
______________________
AR in control group
Calculate OR
Odds of event in exposed
___________________________
Odds of event in control
What’s the difference between OR & RR?
RR - compares RISK
(p) = all events/total pop
OR - compares ODDS
odds = p/1-p
p= 80% (200/250)
Odds = 0.8/0.2 = 4.0
Calculate Incidence rate
New cases
____________
(Pop x time)
Calculate Risk (incidence)
No. new cases
_____________________
Pop x time
Calculate point prevalence
AT ONE TIME POINT:
No. ppl with disease
(new & existing cases)
_______________________
Total pop. at risk
What is period prevalence?
No. new & existing cases over defined period
What is a hazard ratio?
Survival analysis - ratio of survival btw intervention & comparator
Calculate RD (attributable risk)
AR exposed - AR unexposed
How does sample size impact a study’s data?
Bigger sample = greater precision (smaller CIs)
When interpreting data such as RR, what 3 things do you comment on?
-
Size & direction of effect: What the value means in application
RR = 0.75 means a 25% decreased risk
OR = 0.75 means a 25% less likely outcome
RD = 0.75 means a 75% increase in risk - Precision: What the range of the risk could be based on the values given by the CI
- Significance: Whether it is statistically significant and why
How is consistency between studies shown?
The same direction and signifiance of effect but varying size
(E.g. A statistically significant reduction in mortality although point estimates may differ)
NNT
1/RD (round to whole person)
“Incidence rate & risk are the same thing.”
True or false?
True
Define absolute risk
Measure of the risk in a population (ie - risk of outcome in exposed group)
