Home>Domains-Harm/Etiology-Calculations
Calculating Harm/Etiology
Calculators
Centre for Evidence Based Medicine Stats Calculator
Vanderbilt- Power and Sample Size Calculator
Calculations
| Yes |
No |
|
| Exposure | a |
b |
| No Exposure | c |
d |
|
Appropriate use |
Formulae |
Relative Risk |
|
RR= (a/a+b)/(c/c+d) |
Odds Ratio |
|
OR= (a/c)/(b/d)=ad/bc |
When outcomes or events are rare, the estimates of RR are similar to those of OR. As the outcomes become more common, this approximation no longer holds.
An excellent review of Odds Ratios by Jon Deeks may be found on Bandolier. Each search using odds ration or hit this direct link:
http://www.jr2.ox.ac.uk/Bandolier/band25/b25-6.html
Interpreting Strength of Association
For randomized trial, strong association if RR or OR > 1
For cohort study, strong association if RR > 3 or OR > 4
Accept lower strength of association if serious consequence/outcome to exposure
Require higher strength of association if more trivial consequence/outcome to exposure