Class 8-10: Asso. Flashcards
2x2 table is searching for
Associations
Counterfactual theory
The attempt to create all people in the group as equal “IF” the exposure/Tx/disease had NOT occurred…This is impossible to create. Once a smoker has smoked…he has smoked and that can be reversed.
Our best attempt at Counterfactual theory
Exchangability
Exchangeability
Comparability
Label for columns of the 2x2
Disease/outcome
Label for the rows of the 2x2
Yes/No: if exposure/Tx occurred
Absolute Difference
Subtracting frequencies
Relative differences
Division of frequencies
And division of proportions
Which is larger relative or absolute differences
Relative differences are larger
Risk is a frequency/ratio/proportion?
Proportion
Probability of outcome
A simple percentage
Absolute Risk reduction (ARR)
This is the absolute difference of the outcome attributable to exposure difference between groups.
Relative risk reduction (RRR)
ARR / Risk of unexposed
Number needed to Treat (NNT) / Number needed to harm
Attempt to recreate relative risk into whole people… 1 / Absolute risk reduction—This is the number of patients needed to be treated to receive the stated benefit/harm
Risk ratio
Ratio of the 2 risks:
[ Risk of outcome (exposed) / risk of outcome (unexposed)]
All ratios:
Compare 2 groups and describe likelihood of event/outcome in 1 group compared to the other
If the ratios have the ratio of 1.0, then the outcome is:
Equally as likely
If risk ratio is > 1 then the outcome is:
MORE likely to occur than the comparison group
If the risk ratio is
Less likely to occur in the comparison group
If the risk ratio is between 1-1.99 how do we compare to comparison group
Use the decimal value as a %
If the risk ratio is >2 how do we compare to comparison group
‘X’ times greater than: 6.18=6.18 times greater
If the risk ratio is between .00001 - .99 how do we compare to comparison group
Subtract from 1…then convert to %;
0.73 = 27% decreased
When interpreting ratios, there are 3 factors to consider:
- Group (intervention/placebo)
- Direction (increase/decrease)
- Magnitude (%/X)
Odds:
Frequency of an outcome occurring vs. not occurring
Odds Ratio:
[ odds of exposure (diseased) / odds of exposure (non-diseased)]——can also cross multiply
