2-13 Relative measures of effects can be misleading

Relative measures of effects (e.g. the ratio of the probability of an outcome in one treatment group compared with that in a comparison group) are insufficient for judging the importance of the difference (between the probabilities of the outcome). A relative effect may give the impression that a difference is larger than it actually is when the likelihood of the outcome is small to begin with.

For example, if a treatment reduces the probability of getting an illness by 50% but also has harms, and your risk of getting the illness is 2 in 100, receiving the treatment is likely to be worthwhile. If, however, your risk of getting the illness is 2 in 10,000, then receiving the treatment is unlikely to be worthwhile even though the relative effect is the same.

Always consider the absolute effects of treatments – that is, the difference in outcomes between the treatment groups being compared. Do not make a treatment decision based on relative effects alone.

Browse by Key Concept

Back to Learning Resources home

Filter these resources:

Clear Filters

Know Your Chances

This book has been shown in two randomized trials to improve peoples' understanding of risk in the context of health care choices.

| 0 Comments | Evaluated
Logo

Bias

A University of Massachusetts Medical School text on biases.

| 0 Comments

Dodgy academic PR

Ben Goldacre: 58% of all press releases by academic institutions lacked relevant cautions and caveats about the methods and results reported

| 0 Comments

Defining Risk

This blog defines ‘risk’ in relation to health, and discusses some the difficulties in applying estimates of risk to a given individual.

| 0 Comments
Book cover

อย่าถูกหลอกด้วยสถิติที่สะดุดตา

“สมมติว่าการมีคอเลสเตอรอลสูงจะเพิ่มความเสี่ยงกล้ามเนื้อหัวใจตายเฉียบพลันขึ้นร้อยละ 50 ในผู้ที่อายุอยู่ในช่วง 50 ปี ก็ฟังหนักหนาเอาการ แต่ถ้าความเสี่ยงนี้เพิ่มเพียงร้อยละ 2 ก็ฟังไม่เลวร้ายนัก แต่ทั้งสองค่าคือค่าเดียวกัน (ค่าเหล่านี้สมมติขึ้น) ลองคิดดังนี้ ในชายอายุในช่วง 50 ปีจำนวน 100 ราย หากมีคอเลสเตอรอลปกติ คาดว่า 4 […]

| 0 Comments

No Resources Found

Try clearing your filters or selecting different ones.