Absolute and relative risk reduction

Reporting drugs and treatments Thomas Abraham

What we will learn today 



 

The difference between absolute and relative risk reduction A basic way to interpret P values and CI (confidence intervals) Number needed to treat (NNT) All of these are to help you evaluate and report on studies

Nov 1995, results of the trial of a drug announced 





6,595 men in the west of Scotland with elevated cholesterol level participated in trial that lasted 5 years One group (treatment group ) was given a cholesterol lowering drug, pravastatin Another group (control group) was given a placebo

The results were impressive For those who took the drug:  31 % reduction of risk from heart attacks  28 % reduction of risk of death from coronary heart disease  31 % reduction of risk from heart attack, or death from heart disease

Combined risk from heart attacks and death from heart disease

7.9%

5.5%

Risk from heart attack and death from heart disease Control group

A 7.9%

Treatment group

B 5.5%

How do you measure the difference between these two groups?

Absolute and relative risk reduction 



Absolute risk reduction looks at the difference between the treatments: A-B =2.4% Relative risk reduction looks at how much better one treatment is than the other (A-B/ A) x 100=30.37%

A Control 7.9%

B Treatment 5.5%

Absolute and relative risk 





Absolute risk is your risk of getting a particular disease, or condition Relative risk measures how much your risk is reduced with the new treatment, or procedure, compared with the old. Relative risk figures tend to look dramatic

Death from coronary heart disease

1.8 1.6 1.4 1.2

A Control 1.7 % B Treatment 1.2%

1 0.8 0.6 0.4 0.2 0 Series1

Placebo 1.7

1.2

Series1





Absolute risk reduction= Relative risk reduction=

Non fatal Myocardial Infarction

A Control 6.5%

Absolute Risk= ?

B Treatm ent 4.6%

Relative risk= ?





What were the different outcomes the study measured? Which outcome was the treatment the most effective in?

P values and Confidence Intervals ( CI) P value or probability value= The probability that a result could have occurred by chance P < .05 means a less than 5 percent probability that this result could have occurred by chance P< .01 means a less than 1 percent probability that this could have occurred by chance P