Epidemiology

• Ecological Studies


  Studies that investigate risk factors of health outcomes in which the unit of analysis is at the group level rather than the individual
• Group Measures


  • Summary measures of a group (mean, average rate)
  • Environmental factors (air pollution, hours of sunlight)
• Ecological Studies

  • Time trends
  • Geographic comparisons
• Advantages of Ecological Studies

  • Easy to do
  • No individual data necessary
  • Good to generate ideas about potential associations
• Disadvantages of Ecological Studies

  • No information on the individual level
  • Not able to account for other factors that might explain the association
• Ecological Fallacy


  Occurs where an analysis of group data is used to draw conclusions about the individual
• Cross-Sectional Study


  A study in which a sample of persons from a population are enrolled and their exposures and health outcomes are measured simultaneously
• Cross-Sectional Study


  1. Select a sample representing the population of interest

  2. Measure exposure and outcome variables at the same time

  3. Determine prevalence
• Advantages of Cross-Sectional Studies

  • Fast & inexpensive
  • Immediate answers – no follow up time
  • No loss to follow up
• Disadvantages of Cross-Sectional Studies

  • Can't determine temporal relationship
  • Not good for rare exposure or outcomes
  • Bias can be a problem – measurement bias, survivor bias etc.
• Cohort Study


  Observational analytic study in which enrollment is based on status of exposure to a certain factor or membership in a certain group. Populations are followed, and disease, death, or other health-related outcomes are documented and compared.
• Advantages of Cohort Studies

  • Can establish sequence of events
  • Can assess risk of multiple outcomes at the same time
  • Can estimate incidence
  • Able to directly calculate absolute and relative risk

  
• Disadvantages of Cohort Studies

  • Not a controlled experiment so can't claim causation
  • Difficult to control for all other confounding factors
  • Expensive – often require large sample
  • Not good choice for rare outcomes
• Prospective Cohort Study


  Start with assembling a cohort, measure risk factors then follow overtime to measure outcomes
• Advantages of Prospective Cohort Studies

  • Can control who is in the cohort
  • Lower risk of bias
• Disadvantages of Prospective Cohort Studies

  • Not timely, long follow up
  • Potential loss to follow up (can lead to bias)
• Retrospective Cohort Study


  Identify a suitable cohort, collect risk factor data measured in the past, collect subsequent outcome data
• Advantages of Retrospective Cohort Studies

  • More efficient: less time; less costly
• Disadvantages of Retrospective Cohort Studies

  • Knowing the outcome might lead to bias
  • Limited to data already collected
  • Little control over who is in the cohort
• Case-Control Study


  An observational analytic study that enrolls one group of persons with a certain disease and a group of persons without the health problem (control subjects) and compares differences in exposures, behaviors, and other characteristics to identify and quantify associations, test hypotheses, and identify causes.

  
• Case-Control Study


  1. Select a sample of cases (people who have the condition)

  2. Select a sample of controls (people without disease but have same chance of having the disease)

  3. Measure (past) exposure to risk factors of interest

  
• Advantages of Case-Control Studies

  • Useful for rare outcomes
  • Efficient/less costly than cohort study
• Disadvantages of Case-Control Studies

  • Biases if cases and controls come from different populations
  • Biases due to measuring exposure after the outcome
  • Confounding due to. Other influential factors
  • Can only study one outcome
• Internal Validity


  Validity of the inferences as they pertain to members of the source population. Prerequisite for external validity. It implies accurate measurement of effects apart from random errors.
• External Validity

  Validity of the inferences as they pertain to people outside that population.
• Validity


  Lack of Bias. Only unbiased procedures are valid.
• Bias

  Systematic error in the study design that results in an estimate of the association between exposure and outcome that is different from the casual association.
• Sources of Bias


  • Selection of participants
  • Measurement aspects on the selected participants
• Unbiased


  The difference between the expected value of an estimation procedure and the true value that the procedure is attempting to estimate is zero.
• Selection Bias


  Systematic difference in the enrollment of participants in a study that leads to an incorrect result (e.g., risk ratio or odds ratio) or inference. Distortions that result from procedures used to select subjects and from factors that influence study participation.
• The relation between exposure and disease is different for those who participate and those who should be theoretically eligible for study. Because the association between exposure and disease among nonparticipants is usually unknown, the presence of selection bias must usually be inferred, rather than observed.
• Particular problem in case control studies because the selection of cases and controls which takes place after outcome has occurred may be related to exposure.
• Self-Selection Bias


  Considered a threat to validity since the reasons for self-referral may be associated with the outcome.
• Self-Selection Bias


  • The CDC did a study on subsequent leukaemia incidence among troops who had been present at the Smoky Atomic Test in Nevada
  • Healthy worker effect: Healthy people more likely to be working whereas those who remain unemployed, retired, disabled are as a group less healthy.
• Diagnostic Bias

  Can occur if outcome in individuals is more likely to be ascertained as a consequence of a particular exposure.
• Diagnostic Bias


  • OC use and venous thromboembolism. Proportion of the women in the study had been hospitalized for evaluation of this disease because they were currently taking OCs.
• Information Bias


  Once subjects are in study, the information to be compared between groups can lead to bias if there are errors in the measurement of subjects.
• Differential Misclassification


  Classification error that depends on the values of other variables. More of a problem for case control studies because classification of exposure occurs after disease has occurred. Bias can exaggerate or underestimate an effect.
• Recall Bias


  Differential misclassification of exposure. Can occur in case control studies when cases recall their past exposures differently than controls. Amount of time elapsed between the exposure and the recall is an important indicator if the accuracy of recall.

  
• Nondifferential Misclassification

  Classification error that does not depend on values of other variables. Exposure nondiff. misclassification: The proportion of subjects misclassified on exposure does not depend on disease status. Disease nondiff. misclassification: the proportion of subjects misclassified on disease does not depend on exposure.