ExEpi-Unit1

Physical activity epidemiology

The study of the relation between physical activity and health using epidemiologic methods

Benjamin Rush

1770s

“Sermons to gentlemen upon temperance and exercise”

Plan of a federal university

Epidemiology

 the study of the distribution and determinants of health related states or events in specified populations and the application of this study to the control of health problems

Epi

Upon, among

-demos

the populace, the people

-ology

the study of

“study”

Basic science of public health

Quantitative 

Based on principles of stats and research methodologies

“distribution”

Study the distribution of frequencies and patterns of health events

Try to answer who, what, when, where

“determinants”

Attempt to search for causes or factors associated with risk of disease

Try to answer how and why

KEY: DISEASE IS NOT RANDOMLY DISTRIBUTED 

“health related states”

Early epi = infectious disease

Modern epi = whole spectrum 

“populations”

epi deals with groups of people, NOT individuals

“control”

control and prevent health problems

Data steers public health decision making

Development/evaluation of interventions

Morbidity

Prevalence and incidence of disease

Mortality

Incidence of death

Relative risk = 1

Risk is equal (no association)

Relative risk > 1

Risk in exposed group is greater (positive association)

Relative risk < 1

Risk in exposed group is less (negative assocation)

An epidemiologists goal is to answer this question:

Does exposure cause outcome?

Cause

An exposure or characteristic whose presence has led to one or more individuals developing the disease

Theory of casual mechanisms

All pieces of the pie must be present for disease to occur through this mechanism

• disease begins when last mechanism has acted 

• If any piece of the pie can be prevented, the disease CANNOT occur through that mechanism

Prevalence

• Useful for planning or targeting interventions

• Can be point or period 

• A proportion ranging from 0 to 1

• A “snapshot” of the population

• Does NOT estimate risk of disease

Cumulative incidence

• A proportion ranging from 0 to 1

• Represents the probability that an individual will develop the disease over a specified time period 

• IS a measure of disease risk

• Assumption: all at risk individuals followed for entire period

Incidence rate

• Represents the average rate at which a disease develops over a specified time period 

• A true rate, ranges from 0 to infinity 

• Accounts for differing lengths of follow up 

• Change in incidence reflects change in etiologic factors

Randomized control trial

subjects are randomly assigned to receive/not receive an experimental treatment, procedure, or other intervention

• Gold standard method of testing hypotheses

Randomized control trial PROS

• Strongest evidence of cause/effect

• Allow standardization of selection, exposure, study measures

Randomized control trial CONS

• Possibly expensive 

• Not suited to many research questions

• May not be generalizable

Prospective cohort

• Begin with a group of people (cohort) free of disease

• Classified according to exposure to a potential cause of disease

• Exposed and unexposed subjects followed over time to determine whether they experience the outcome/disease

Prospective cohort PROS

• Exposure measured before disease occurs 

  • Temporal sequence established 

  • Exposure measurement less prone to bias

• Can establish incidence of disease

• Can assess more than 1 outcome

Prospective cohort CONS

• Large #’s needed for rare outcomes 

• Often long time frame

• Expense

Case control

• starts with the identification of people with the disease of interest and a suitable control group of people without the disease

  • Compare exposure among persons with the disease (cases) and among persons without the disease (controls)

Case control PROS

• Good for studying rare conditions 

• Relatively small # of subjects

• Relatively inexpensive and short duration

Case control CONS

• Limited to 1 outcome 

• Prone to bias

• Temporal sequence of events not certain 

• NOT useful for studying rare exposures

Cross sectional

• survey of a sample of the population in which the status of individuals with respect to 1 or more characteristics is assessed at 1 point in time 

  • Measures the prevalence of disease

  • Hypothesis generating

Cross sectional PROS

• Can measure more than 1 exposure and outcome 

• Usually of short duration 

• Yield prevalence estimates

Cross sectional CONS

• Cannot establish temporal sequence of events 

• Do NOT yield incidence or true RR

• NOT good for rare exposures or outcomes

Odds ratio

Only measure of association available for case control

Goals of analytic study

• Obtain an accurate measure of association between the exposure and outcome of interest 

• Determine whether the exposure is independently (and possible casually) related to the outcome

Confounder effects

• Can create the appearance of a cause effect relationship when none exists or mask true associations 

• Distortion of results by a confounder can be large or small

• Can increase/decrease the apparent association between an exposure and disease

Methods to control confounding

• In the design they include: 

  • Randomization, restriction, matching 

• In the analysis they include: 

  • Stratification and statistical modeling

To be accurate, must minimize

Random error and systemic error

Estimates can go astray

randomly and systematically

Random error

• the divergence, due to chance alone, of an observation on a sample from the true population value

• Affects precision

Random error sources

Individual biological variation

Sampling error

Measurement error 

Systematic error

• occurs when results are produced that systematically depart from the true population values 

• Affects validity

Systematic error may arise due to methods used to

• Select study participants 

• Collect info regarding exposure and outcome

Selection bias

Results from flaws in how study participants are selected

Info bias

Results from flaws in how exposure or outcome info obtained