B4

Goals of Epidemiologic Studies

• Clarifies clinical and demographic characteristics.
• Identifies who is at risk for illness.
• Provides clues to causes of disease.
• Guides preventive measures and interventions.

Major Types of Epidemiologic Studies

There are two basic categories of epidemiologic studies used to examine health-related characteristics and distributions in human populations:

• Descriptive Epidemiology: Involved with the amount and distribution of disease or other health-related characteristics by person, place, and time. It identifies non-random variations in health, injury, or disease distribution.
• Analytic Epidemiology: Designed to examine associations and usually begin with a hypothesis of a causal relationship. These involve comparison groups and are concerned with identifying or measuring the effects of risk factors or specific exposures.

Descriptive Epidemiologic Study Types

• Case Reports: A detailed description of one or every few cases that present with an unusual presentation or a specific complication.
• Case Series: A detailed description of a collection of patients or cases who share common characteristics related to clinical features of a disease or condition.
• Correlational or Ecologic Studies: A study where the units of comparison are populations rather than individuals. A limitation is that conclusions drawn from these studies may not apply directly to individuals.
• Incidence Study: Describes the incidence, or number of new cases f a disease or condition, during a specific time within a specific population. A disadvantage is that these studies allow the calculation of true rates of disease occurrence for better estimation of risk.
• Cross-sectional or Prevalence Surveys: Measure the number of events in a given population at a specific point in time. Exposure and outcome data are collected simultaneously.

Analytic Epidemiologic Study Types

Analytic studies are categorized based on how subjects are assigned and observed:

Experimental Studies

These use a randomized selection process based on chance to assign subjects to exposure groups. They include:

• Clinical Trials: Tests a preventive or therapeutic procedure or intervention.
• Preventive Trials: Designed to evaluate if an intervention reduces the risk of developing a disease in those currently without the condition.
• Community Intervention Trials: Uses an entire community as the unit of intervention to measure predefined outcomes.

Observational Studies

These are non-randomized designs where the investigator simply observes patterns of exposure as they occur naturally rather than assigning exposures. They include:

• Case-Control Studies: Compares a group with a disease (cases) to a group without (controls) to see how frequently a suspected risk factor is present in each.
• Cohort Studies: Follows two groups (one exposed, one unexposed) over time to compare outcome frequency.
• Cross-sectional or Prevalence Surveys: Used here to examine relationships between variables and disease.

Case-Control Study Characteristics

In a Case-Control study, the investigator identifies diseased individuals (Cases) and non-diseased individuals (Controls). Both groups are then evaluated for their exposure status (Exposed or Unexposed).

• Advantages:
  • Particularly useful for studying rare conditions.
  • Relatively cheap to conduct.
  • Allows for rapid investigation and hypothesis testing.
  • Enables the testing of multiple exposures against a single outcome.
• Disadvantages:
  • Subject to recall bias.
  • Requires strict control for confounding bias.
  • Difficult to establish correct temporal relationships because data is retrospective.
  • Inability to estimate incidence rates of disease.

Cohort Study Characteristics

Cohort studies follow groups based on exposure status. These can be prospective (exposure recorded before outcome) or retrospective (outcomes already occurred, but orientation remains exposure-based).

• Advantages:
  • Exposure is documented to precede outcomes.
  • One exposure can be linked to multiple potential outcomes.
  • Allows determination of actual incidence rates.
  • Efficient for studying rare exposures.
  • Retrospective cohorts are faster and less expensive than prospective ones.
• Disadvantages:
  • Requires extensive resources.
  • Requires a large number of subjects.
  • Necessitates long follow-up periods.
  • High risk of subjects dropping out before completion.
  • Difficult to obtain accurate documentation on exposure status.

Cross-Sectional (Prevalence) Study Details

These studies measure prevalence—the number of events in a population at one time. They are easy and economical but have specific limitations.

• Limitations:

  1. Unlikely to reveal health events of short duration (rapid recovery or mortality). Associations found typically involve long-duration outcomes.
  2. Results are not automatically applicable to the source population; size and composition of the study population must be assessed first.

• Calculations and Formulas:

  • Prevalence Ratio ($PR$): Comparison of disease prevalence in exposed vs. unexposed groups.     $PR = \frac{a / (a+b)}{c / (c+d)}$
  • Prevalence Odds Ratio:     $\text{Prevalence odds ratio} = \frac{(a)(d)}{(c)(d)}$

• Data Matrix Template:

  • Exposed with disease: $A$
  • Exposed without disease: $B$
  • Unexposed with disease: $C$
  • Unexposed without disease: $D$

Preferred Ratio Measures by Study Design

• Cross-sectional Study: Prevalence ratio; Prevalence odds ratio.
• Cohort Study: Risk ratio; Rate ratio.
• Case-Control Study: Odds ratio.

Experimental Study Design and Blinding

Experimental studies must meet a basic design: a hypothesis must exist, there must be at least two similar groups (one control, one experimental), and subjects must be randomly assigned.

• Types of Blinding:
  • Single Blinded: Subjects do not know if they are receiving the exposure/intervention.
  • Double-Blinded: Neither the subjects nor those assessing the outcomes (investigators) know who is receiving the exposure.
  • Triple Blinded: Subjects, outcome assessors, and those assigning subjects to groups are all unaware of the exposure status.
• Advantages:
  • Exposure documented to precede outcomes.
  • One exposure can test many outcomes.
  • Randomization controls known and unknown factors.
  • Masking (blinding) minimizes bias.
• Disadvantages:
  • Requires extensive resources, large subject numbers, and long follow-up.
  • Potential for high dropout rates.
  • Involves numerous ethical issues.
  • Requires large-scale coordination.

The 5 W’s of Epidemiologic Studies

Epidemiological inquiries are organized around five questions focusing on specific information types:

• What: Subject / Information.
• Who (Person): Descriptive Epidemiology.
• Where (Place): Descriptive Epidemiology.
• When (Time): Descriptive Epidemiology.
• Why / How: Causes (Descriptive); Risk factors and Modes of transmission (Analytic Epidemiology).