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how do you calculate NNT

1/ ARD


how do you calculate Absolute risk reduction

risk(group1)- risk (group2)


how do you calculate relative risk ratio

risk (group1)/ risk (group2)


what are odds ratios used for

case control studies or observational studies and regression models


how do you calculate odds ratios

number of smokers/ number of non smokers = odds of being a smoker

odds ratio = odds smoker if case/ odds smoker if control

=odds of being a smoker if you died by 60 are 6 times the odds of being a smoker in the group who lived


how do you calculate standard deviation

sum of squared deviations from the mean / n-1

square root the answer


what is standard error of the mean

the variability of means from different samples

a large SE indicates that there is much variability in sample means

SE is always smaller than SD because there is less variability between sample means than individual values
can also calculate the standard error of odds ratio

larger samples lead to a smaller SE


how do you calculate SE

SE= SD / sq root of n


what is a confidence interval

gives a range of values, estimated from the sample data which is likely to include the mean

a range of values within which we are 95% confident the true population mean lies
tells us whether observed differences are likely to be due to chance alone or a true effect