1. Risk Different ways of assessing it

2. Objectives Be able to define and calculate: Absolute risk (reduction) Relative risk (reduction) Number needed to treat (NNT) Number needed to harm (NNH) Odds and odds ratio

3. Definitions in words Absolute risk Absolute risk A measure of the chances of an event occurring in a group Absolute risk reduction A measure of the difference between the absolute risk of events in the experimental group from the absolute risk in the control group (arithmetically this is a simple subtraction).

4. Definitions in words Relative risk Relative risk The ratio (comparison) of an event occurring in one group compared to another. For instance: the risk of acute asthma attacks for patients on beclamethasone inhaler compared to those only on salbutamol.

5. Definitions in words Relative risk Relative risk reduction The percentage reduction in the risk of an event in the group on the experimental regimen compared to another group (usually control group). For instance: the percentage reduction in acute asthma attacks in patients on beclamethasone, compared to those only on salbutamol (the control group).

6. Definitions in words Numbers needed to treat (NNT) Numbers needed to treat (NNT) NNT is the number of patients that need to be treated to achieve one event or result. For instance: number of patients that need to be treated with beclamethasone to prevent one acute asthma attack.

7. Definitions in words Numbers needed to harm (NNH) Numbers needed to harm (NNH) NNH is the number of patients that receive a treatment for one to suffer an adverse event.

8. Definitions in words Odds Odds Odds are the ratio of the successful occurrence of an event compared to the failure of that event to occur. Odds are not the same as probability.

9. Definitions in words Odds For instance, if you draw cards from a pack at random: On 13 occasions you will draw a spade But on 39 occasions you will not draw a spade. The event, ‘drawing a spade’, successfully occurs on 13 occasions But is unsuccessful on 39 occasions. So, the odds of drawing a spade from a pack of cards are 13:39, or 1:3.

10. Definitions in words probability The probability of drawing a spade 13:52, or 1:4. However, in the statistical sense, probability is a complicated concept. It is often expressed as the p value

11. Definitions in words Odds ratio Odds ratio Odds ratio is the ratio of two odds (i.e., comparing the odds of one event occurring compared to the odds of another event occurring).

12. Definitions in words Odds ratio The odds ratios range from 0-1. In clinical trials, almost always, the lower the odds ratio, the more beneficial is the experimental regimen. For instance, the odds of having an acute attack of asthma on beclamethasone are lower than on salbutamol alone.

13. Definitions in words Odds ratio Odds ratio of likelihood of acute asthma attacks on beclamethasone: salbutamol is bound to be <1. and the fewer the attacks of asthma on beclamethasone, lower the odds ratio.

14. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Risk - The maths Generic table

15. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Absolute risk AR newcare = a. a + b AR control = c. c + d

16. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Absolute risk reduction AR control -AR newcare

17. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Relative risk (RR) AR newcare ÷ AR control

18. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Relative risk reduction (RRR) RRR newcare =AR control – AR newcare AR control

19. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d NNT to prevent one event Use Absolute risk reduction as the fraction, not the percentage =1 Absolute risk reduction

20. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Odds newcare=a b Odds control=c d

21. Event occurs Event does not occur Total New careaba + b Usual care/control cdc + d Totala + cb + da + b +c +d Odds ratio= Odds newcare. Odds control