8 - Relative & Absolute Risk

Question 1

Formula for relative risk

Answer 1

• RR = probability of an event in tx group / probability of an even in control group
• Probability of event = # of px experiencing the event / total # in either tx or control group

Question 2

Formula for relative risk reduction

Answer 2

RRR = 1 - RR

Question 3

What is the purpose of a confidence interval?

Answer 3

• Purpose is to give a reasonable estimate where the population statistic is likely to be based on the sample estimate
• If the tx works, we expect RRR to be less than 1, so the confidence interval shouldn’t contain 1
• If we expect 2 things to be different, we don’t want the CI to contain 0

Question 4

What is NNT?

Answer 4

• Number of patients who needed to be treated in order to prevent one additional bad outcome
• 1 / ARR (rounded to nearest whole number)