Understanding real research 1.

1. Understanding real research 1. Cohort studies.

2. Epidemiological questions. How common is the problem? What are the risks of further problems? Are treatments effective? What are the causes of the problems described? How effective are services?

3. Underpinning concepts. What concepts underpin these types of questions? Likelihood Risk Effectiveness Association Population level data applied to individuals

4. What can studies do? Describe the situation: Descriptive. Explain the situation: Analytical. Compare approaches: Experimental.

5. Study designs. • Descriptive • Cross-sectional, longitudinal. • Analytic • Case-control studies. • Cohort studies. • Quasi-experimental • Natural experiments, policy interventions. • Experimental • Randomised controlled trial.

6. Study designs

8. Past Present Future Cross-sectional Case-control Cohort From Altman. Practical Statistics for Medical Research, 1991.

9. Randomised controlled trial Sample Randomise Intervention group Control group

10. Cohort studies: Key features. Prospective, i.e. forward looking. Follow a population sample over a period of time, often years. Record new disease events, i.e. the disease incidence. Cohorts can be selected in either of two ways.

11. Selection of a cohort: External controls. • A cohort of individuals that have been exposed to a postulated causal factor of disease. The cohort is followed up and all new cases of the disease recorded. In this type of study, the exposed group would be compared to a matched, unexposed group and the incidence of disease in the two groups compared, i.e. an external control. • An example would be a study to record the incidence of mesothelioma in factory workers exposed, or not, to asbestos.

12. Cohort studies. From: http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html

13. Selection of a cohort: Internal controls. A population cohort can be selected and followed up over many years to determine the incidence of disease(s). In this type of study, those in the cohort who do not develop the disease act as the controls for those who do, i.e. an internal control. An example would be the MIDSPAN study.

14. Cohort studies. From: http://www.socialresearchmethods.net/tutorial/Cho2/cohort.html

15. Advantages & disadvantages

21. Appraising cohort studies Methodological approach: Representativeness of the inception cohort. Characterisation at study baseline. Nature and completeness of follow-up. Statistical reporting: Type of data – influences statistical analysis. Reporting of risk.

22. Reporting risk Absolute risk. Relative risk. Population attributable risk.

23. 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.

24. Absolute risk 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 = Rate of occurrence in exposed group – Rate of occurrence in unexposed group.

25. 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.

26. 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% = Total population exposed (Incidence in exposed population – Incidence in unexposed group)/Incidence in total population All multiplied by 100% to give a percentage