VPH 121 - OBSERVATIONAL STUDY

Experimental study

researcher assigns the exposures

Observational study

the researcher did not assign the exposures

Randomized controlled trial

the researcher assigns the exposure and the allocation is random

Non-randomized controlled trial

the researcher assigns the exposure and the allocation is not random

Analytical study

the researcher did not assign the exposures

comparison is present

Descriptive study

the researcher did not assign the exposures

comparison is not present

Case-control

Cohort study

Cross sectuibak

Give the 3 types of analytical study

Case report and case study

2 components of descriptive study

Case report

• Focus on individual animal with “newsworthy” clinical occurrence

• Qualitative > quantitative

Case report - 1

A cartilaginous choristoma in a pig liver: a case report

Case report - 2

Silent histomoniasis on a brooder farm

Case series

Multiple occurrences of an unusual case

Case series - 1

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

Case series - 2

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

Higher prevalence in exposed

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

Higher incidence in exposed

What are the first three rules of Evan that are used in assessing causation in an analytical study?

Cohort study

• Most effective in assessing causal hypotheses

• prospective and retrospective

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

Advantages 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

Disadvantages of cohort study

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?

RR>1

high likelihood of association

RR=1

no association

RR <1

not an exposure, protective effect

Attributable risk

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

AR > 0

excess absolute risk

AR = 0

absence of additional risk

AR < 0

not a risk factor, protective factor

Case control study

• selects diseased and disease- free populations

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

• 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

OR > 1

highly associated

OR = 1

not associated

OR < 1,

less association

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