Trong nghiên cứu y tế, tỷ lệ tỷ lệ cược được sử dụng thường xuyên cho các trường hợp nghiên cứu kiểm soát và nghiên cứu hồi cứu bởi vì nó có thể được lấy dễ dàng hơn và với chi phí ít hơn hơn so với các nghiên cứu mà phải ước tính tỷ suất mới mắc trong các nhóm nguy cơ khác nhau | 16 Association Cause and Correlation 287 times higher than the odds with B. In medical research the odds ratio is used frequently for case-control studies and retrospective studies because it can be obtained easier and with less cost than studies which must estimate incidence rates in various risk groups. Relative risk is used in randomized controlled trials and cohort studies but requires longitudinal follow-up and thus is more costly and difficult to Relative Risk Reduction RRR and Absolute Risk Reduction ARR and Number Needed to Treat NNT The RRR is simply 1 - RR times 100 and is the difference in event rates between two groups . a treatment and control group . Let s say you have done a trial where the event rate in the intervention group was 30 100 and the event rate in the control group was 40 100. The RRR is 25 . 10 absolute reduction divided by the events in the control group of 10 40 . The absolute risk reduction ARR is just the difference in the incidence rates. So the ARR above is minus or a difference of 10 cases. But what if in another trial we see 20 events in the control group of size N vs. 15 in the intervention group of size N The RRR is 5 20 or 25 while the ARR is only 5 . Absolute risk reduction ARR is another possible measure of association that is becoming more common in reporting clinical trial results of a drug intervention. Its inverse is called the number needed to treat or NNT. The ARR is computed by subtracting the proportion of events in the control group from the proportion of events in the intervention group. NNT is 1 ARR and is a relative measure of how many patients need to be treated to prevent one outcome event in a specified time period . If there are 5 100 outcomes in the intervention group say you are measuring strokes with BP lowering in the experimental group over 12 months of followup and 30 100 in the control group the ARR is - and the NNT is 4 1 that is for every four .
