VPH 121 LE 2 Lec 1: Observational Study

Observational Study

Observational Study

Conducted to observe exposures that are not assigned by the researcher

Definition: Causation

Diseases do not occur randomly

prove causation by:

1. describe chain of events from cause to effect

2. study at a molecular level

Assessing association

1. assessment of exposure and risk indicator

2. calculation of risk or odds of having a disease

Assessing causation

calculation of risk or odds of having a disease

Factors in calculating the risk or odds of having a disease

1. predictive

2. diagnostic

3. management

Goals of Epidemiology

1. describe the health status of a population

2. explain causality or association

3. predict disease occurrence

4. control the spread of diseases

Types of Descriptive Study

1. Case Report

2. Case Series

Case Report

• single occurrence of an unusual case

• qualitative > quantitative

Case Series

• multiple occurrences of an unusual case

Types of Analytical Study

1. Cohort

2. Case Control

3. Cross-sectional

First three rules of Evan

1. Higher prevalence in exposed

2. Exposure to a cause should be present more in disease population

3. Higher incidence in exposed

Most effective study in causal hypotheses

Cohort study

two types of cohort study

1. prospective

2. retrospective

Advantages of Cohort Study

1. Calculation of incidence

2. Flexible in choosing variables

3. Investigate multiple outcomes and potential risk factors

4. Rare exposures

5. Progression of the disease

6. Temporal cause and effect relationship

7. Second choice if experimental studies are unethical

Disadvantages of Cohort Study

1. Sampling error

2. Large population size for rare diseases

3. Long duration of follow-up

4. Difficulty in follow-up

5. High cost

6. Confounding variables

Relative Risk

• Ratio between cumulative incidence

• Risks between the exposed and unexposed groups

Relative Risk

• RR < 1

• RR = 1

• RR > 1

• not an exposure; protective effect

• no association

• high likelihood of an association

Attributable Risk

Absolute measure of excess risk in the exposed from the unexposed group

Attributable Risk

• AR < 0

• AR = 0

• AR > 0

• not a risk factor; protective factor

• absence of additional risk

• excess absolute risk

Type of study used when studying diseases with low incidence and conditions with long follow-ups

Case Control Study

Case Control Study

• Selects diseased and disease-free populations

• no measure of disease frequency

Advantages of Case Control Study

1. Rare diseases

2. Diseases with long incubation or latent periods

3. Fast conduct

4. Low cost

5. Few subjects are required

6. Available records

7. Absence of risk

8. Multiple exposures

Disadvantages of Case Control Study

1. Sampling error

2. Poor quality of records

3. Difficulty in validation

4. No control on variables

5. Selecting control group

6. No assessment of temporal sequence

7. Rare exposures

8. Limited to one outcome

Odds Ratio

• Ratio between odds of disease in exposed and unexposed groups

Odds Ratio

• OR < 1

• OR = 1

• OR > 1

• less association

• not associated

• highly associated

Cross-Sectional Study

• Random selection and examination of a population at one point in time

• Describing disease occurrence at the time of collection

• Challenge in investigating causal hypotheses

Advantages of Cross-sectional Study

1. Estimation of prevalence or positivity rate

2. Fast conduct

3. Moderate cost

4. Records can be used occasionally

5. No risk to subjects

6. Multiple exposures and outcomes

Disadvantages of Cross-sectional Study

1. Rare diseases and exposures

2. Diseases with short duration

3. Uncontrolled extraneous variables

4. Estimation of incidence

5. Temporal pattern

6. Poor quality of records

Flowchart for Choosing Analytical Study