LECTURE 7: OBSERVATIONAL STUDY

Causation

Diseases do not occur randomly

1. Describe chain of events from cause to effect 2. Study at molecular level

How to prove causation

Calculation of risk or odds of having a disease

Assessment of exposure x risk indicator (assessing association)

1. Predictive 2. Diagnostic 3. Management

Calculation of risk or odds of having a disease (assessing causation)

1. Describe the health status of a population 2. Explain causality or association 3. Predict disease occurrence 4. Control the spread of diseases

Goals of epidemiology

1. Case report 2. Case series

2 types of descriptive study

Case Report

Focus on individual animal with "newsworthy" clinical occurrence

Qualitative

Case report is more of ________

Case series

Multiple occurrences of an unusual case

Case report

Example: A cartilaginous choristoma in a pig liver: a case report

Case report

Example: Silent histomoniasis on a brooder farm

Case series

Example: Rickets: Case Series and diagnostic review of hypovitaminosis D in swine

Case series

Example: Idiopathic Eosinophilic Pneumonia with Associated Pulmonary Vasculitis in Horse: A Case Series

Analytical study

Assess causation using the first three rules of Evan

Higher prevalence in exposed

1st rule of Evan

Exposure to a cause should be present more in disease population

2nd rule of Evan

Higher incidence in exposed

3rd rule of Evan

Cohort Study

Best used in population with similar characteristics

Cohort Study

Most effective in assessing causal hypotheses

Cohort Study

Prospective and retrospective

Cohort Study

Example: investigate the effect of spaying in urinary incontinence in female dogs

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 and Attributable risk

In cohort study, the computed values is/are

Relative Risk

Ratio between cumulative incidence or risks between the exposed and unexposed groups

How many times more (or less) likely are exposed individuals to get the disease compared to the unexposed individuals?

high likelihood of association

RR>1 is interpreted as

no association

RR=1 is interpreted as

not an exposure, protective effect

RR

Attributable Risk

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

excess absolute risk

AR>0 is interpreted as

absence of additional risk

AR=0 is interpreted as

not a risk factor, protective factor

AR

Case Control Study

From outcome, no retrospective

Case Control Study

Selects diseased and disease- free population

Case Control Study

Effective in studying diseases with low incidence and conditions with long follow-up

Case Control Study

No measure of disease frequency

Case Control Study

Example: Investigate the risk factors for those with urinary incontinence

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

In case control study, the computed values is/are

Odds ratio

Ratio between odds of disease in exposed and unexposed groups

highly associated

OR>1 is interpreted as

not associated

OR=1 is interpreted as

less association

OR

Cross-sectional Study

Measures the prevalence

Cross-sectional Study

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

Cross-sectional Study

Describing disease occurrence at the time of collection

Cross-sectional Study

Challenge in investigating causal hypotheses

Cross-sectional Study

Example: How many developed urinary incontinence in dogs from Batong Malake? What are possible risk factors?

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

Descriptive Study

Presence of unusual cases