Chapter 2Nature of the evidence

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

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)

Types of study in physicalactivity and health

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 epidemiological study

Category Type of study Unit of studyObservational

Descriptive studies Case reports or series Individuals

Analytical studies Correlational Populations

Cross-sectional surveys

Individuals

Case-control studies Individuals

Cohort studies Individuals

Experimental Randomized, controlled trials

Individuals

Examples of disease outcomes

Simple• Disease present

• Disease absent

Graded• Normal weight

• Overweight

• Obese

Cholesterol and mortality from CHD in the Seven Countries Study

Harvard Alumni Study

• cardiovascular diseases;• longevity;• diabetes;• gallbladder disease;• several site-specific; cancers;• Parkinson’s disease;• depression;• suicide.

A cohort study that began in 1962, has studied a range of health outcomes, including:

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.

Calculation of person–timeincidence rate

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).

Vigorous sports and attack rate of CHD in English civil servants

Episodes of vigorous sport in previous four weeks, reported in 1976

CHD cases

Man-years of observation

Age-standardized rate (cases per 1,000 man-years)

None (reference group) 413 72,282 5.8

1–3 37 7,786 4.5

8– 12 7 3,349 2.1

All-cause mortality risk among Harvard Alumni 1962–78

Characteristic Prevalence(% of man-years)

Relative risk Population-attributable risk (%)

Sedentary lifestyle

62.0 1.31 16.1

Hypertension 9.4 1.73 6.4

Cigarette smoking 38.2 11.76 22.5

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.

..

Recall of lifetime participation in physical activity

A randomized, controlled, laboratory-based intervention study

Importance of control group: effect of training on heart rate

Accuracy and precision I

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.

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.

Epidemiology and causality?

• 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

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.