Download - Chapter 2 Nature of the evidence
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Chapter 2Nature of the evidence
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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
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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)
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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
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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
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Examples of disease outcomes
Simple• Disease present
• Disease absent
Graded• Normal weight
• Overweight
• Obese
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Cholesterol and mortality from CHD in the Seven Countries Study
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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:
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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.
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Calculation of person–timeincidence rate
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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).
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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
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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
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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.
..
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Recall of lifetime participation in physical activity
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A randomized, controlled, laboratory-based intervention study
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Importance of control group: effect of training on heart rate
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Accuracy and precision I
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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.
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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.
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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
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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.