1. Understanding real research 3. Assessment of risk.

2. What is risk? The probability or chance that an event will occur e.g. the risk of contracting a disease. the risk of dying. the risk of adopting a healthy behaviour (smoking cessation). Risk can be calculated in Case control studies. Cohort studies. Randomised controlled trials.

4. How is risk reported? Relative risk Absolute risk Cohort studies RCTs Odds ratios Absolute risk Case control studies

5. Two by two table

6. Risk A proportion: Numerator / Denominator. Risk of disease in exposed group = a a+b Risk of disease in unexposed group = c c+d

7. Reporting risk in cohort studies Relative risk. Absolute risk.

8. Rates Risk generally expressed as a rate. Rate = Numerator/Denominator Rate = Number of events in a given period e.g. deaths x 1000 Average population over that period Risk of death per 1000 people per year.

9. Beaglehole et al, page 18: Smoking and incidence rate of stroke Smoking category No. of stroke cases Person years of observation Stroke incidence rate per 100,000 person years Never smoked70395,59417.7 Ex-smoker65232,71227.9 Current smoker139280,14149.6 Total274908,44730.2

10. Relative risk Ratio of the risk of an event, e.g. death, amongst those exposed to the risk compared to those unexposed. Ratio = Risk of event in exposed group Risk of event in unexposed group Indicates strength of the association between the agent of interest and the outcome event.

11. RR = 1.0, the risk of the event is same in both groups. RR > 1.0, the group in the numerator is at greater risk of the event. RR < 1.0, the group in the numerator is at lower risk of the event. Remember to consider: Is the event a “good” event or a “bad” event.

12. Beaglehole et al, page 18: Smoking and incidence rate of stroke Smoking category No. of stroke cases Person years of observation Stroke incidence rate per 100,000 person years Never smoked70395,59417.7 Ex-smoker65232,71227.9 Current smoker139280,14149.6 Total274908,44730.2

13. Beaglehole et al, page 18: Smoking and incidence rate of stroke Smoking categoryStroke incidence rate per 100,000 person years Relative risk of stroke Never smoked17.71.0 Ex-smoker27.91.6 Current smoker49.62.8 Relative risk of stroke associated with smoking

14. Absolute risk difference Absolute difference in rates of occurrence between the exposed and unexposed groups. Helps to identify the extent of the public health problem caused by exposure to the agent of interest. Absolute risk difference = Rate of occurrence in exposed group – Rate of occurrence in unexposed group. Also known as attributable risk of exposure or excess risk.

15. Beaglehole et al, page 18: Smoking and incidence rate of stroke Smoking categoryStroke incidence rate per 100,000 person years Absolute risk difference of stroke per 100,000 person years Never smoked17.7- Ex-smoker27.910.2 Current smoker49.631.9 Absolute risk difference of stroke associated with smoking

16. Population attributable risk Incidence of a disease in a population associated with (or attributable to) exposure to a risk factor. Usually expressed as a percentage. PAR% = Incidence in total population – Incidence in unexposed group x100 Incidence in total population

17. Beaglehole et al, page 18: Smoking and incidence rate of stroke Smoking categoryStroke incidence rate per 100,000 person years PAR of stroke associated with smoking Never smoked17.7- Current smoker49.6 Total30.241.4% Population attributable risk of stroke associated with smoking

18. Doll et al BMJ 2004. Can we calculate Relative risk. Absolute risk difference. Population attributable risk. of smoking associated with mortality? This is about getting from Table 1 to Table 3. N.B. We don’t have the data to calculate population attributable risk – no data on overall risk of total population.

19. Reporting risk in case control studies Odds ratio.

20. Rates Remember: Risk generally expressed as a rate. Rate = Numerator/Denominator Rate = Number of events in a given period e.g. deaths x 1000 Average population over that period Risk of death per 1000 people per year. Problem with case control: no denominator population.

21. Risks and odds Risk Is a proportion. Numerator / Denominator. Odds. Ratio. Numerator / (Denominator – Numerator).

22. Two by two table

23. Odds A ratio: Numerator/(Denominator-Numerator). Odds of event in cases = a b. Odds of event in controls = c d.

24. Odds ratio (OR) is: a ratio of ratios. OR = Odds of event in cases = (a/c) Odds of event in controls (b/d) OR = ad bc. Odds ratio

25. Beaglehole et al, page 38: Meat eating & enteritis necroticans Meat eaterDiseaseTotal Yes (Cases)No (Controls) Yes501666 No114152 Total6157118 Risk of meat eating & disease OR of ent necoticans associate with meat eating = (50x41) / (16x11) = 11.6.

26. Risk of VT from oral contraceptives Overall risk, expressed as odds ratio, of VT: All contraceptives vs none: 4.0. Second generation vs none: 3.2. Third generation vs none: 4.8.

27. Absolute risk difference of VT associated with smoking But, what was the absolute risk of death: No contraceptive use: 5 per million users per year. Second generation pill: 14 per million users per year. Third generation pill: 20 per million users per year. Thus, absolute risk difference is: Second generation pills: 9 deaths per million users per year. Third generation pills: 15 deaths per million users per year. What is risk associated with pregnancy??