Chapter 2 Nature of the evidence

1. Chapter 2Nature of the evidence

2. Chapter overview • Introduction • What is epidemiology? • Measuring physical activity and fitness in population studies • Laboratory-based research • Error: nature, sources and implications • Establishing causality • Summary

3. Epidemiology is … ‘the study of the distribution and determinants of health-related states or events in specified populations and the application of this study to control of health problems’. (World Health Organization)

4. Epidemiological Mainly observational Allow nature to take its course and analyse relationships between indices of health status and other variables Laboratory-based Mainly experimental Intervene to see what happens to some/all individuals Types of study in physicalactivity and health

5. Types of epidemiological study

6. Simple Disease present Disease absent Graded Normal weight Overweight Obese Examples of disease outcomes

7. Cholesterol and mortality from CHD in the Seven Countries Study

8. cardiovascular diseases; longevity; diabetes; gallbladder disease; several site-specific; cancers; Parkinson’s disease; depression; suicide. Harvard Alumni Study A cohort study that began in 1962, has studied a range of health outcomes, including:

9. Measures of occurrence of health-related outcomes • Prevalence: the proportion of individuals in a population that exhibits the outcome of interest at a specified time. • Incidence: the number of new occurrences of an outcome that develop during a specified time interval. Best measure is person–time incidence rate.

10. Calculation of person–timeincidence rate

11. Comparisons of disease occurrence between exposed and unexposed groups • These are essential tools in epidemiology. They include: • risk difference; • relative risk; • population-attributable risk; • odds ratio (similar to relative risk, used in many case-control studies).

12. Vigorous sports and attack rate of CHD in English civil servants

13. All-cause mortality risk among Harvard Alumni 1962–78

14. Measuring physical activity/fitness • Job classification; • leisure-time activity; • questionnaire • pedometer • accelerometer • total energy expenditure by doubly labelled water • fitness • direct VO2max (treadmill or cycle ergometer) • predict VO2max from sub-maximal heart rate • functional measure, e.g. time to exhaustion, watts achieved, level in shuttle walking test. . .

15. Recall of lifetime participation in physical activity

16. A randomized, controlled, laboratory-based intervention study

17. Importance of control group:effect of training on heart rate

18. Accuracy and precision I

19. Accuracy and precision II • Data are accurate if they are close to the true values; and • precise if the same measurement, when repeated, consistently yields similar values.

20. Cause and effect • The role of chance, random error: • sampling • measurement. • Bias – systematic error: • subject selection • measurement. • Confounding: • observed association is due to a third factor related to the exposure that independently affects the risk of developing the disease – a confounding variable.

21. Appropriately sequenced; measure of activity/fitness must precede onset of disease Biologically plausible, i.e. is association consistent with other knowledge? Strength – relative risk Dose–response Reversibility Strong study design Consistency in different populations Epidemiology and causality?

22. Summary • Epidemiology can identify risk factors. • In epidemiology, physical activity is most commonly measured by questionnaire. • Relative risk estimates the strength of an association with a risk factor. Associations may reflect the true effect of an exposure, but may also reflect chance, bias or confounding. • Laboratory studies can achieve excellent control and precision and indicate potential mechanisms, but are removed from clinical endpoints. • The totality of the evidence (epidemiology and laboratory-based) determines decisions as to causality.