VPH 121: Observational Study (Eljon)

What is causality?

1. diseases do not occur randomly; thus, there is causation

2. causation includes temporal, spatial, health status of the population

How do we prove causation?

1. describe chain of events from cause to effect

2. study at molecular level

Steps on assessing association

1. assessment of exposure multiplied by risk indicator

2. calculation of risk or odds having a disease

Assessment of exposure

1. when checking exposure: look at outcome

2. when checking outcome: look at exposure

Risk indicators

1. risk ratio

2. odds ratio

Risk ratio

1. an example is the cumulative incidence

2. the first step is to observe the population, and then check the outcome of the exposure for new cases

Odds ratio

1. starts with disease presence

2. there no more risk of developing the disease

Assessing causation

calculates the risk/odds of having a disease

Predictive

1. at risk of disease development due to increased susceptibility

2. for example: CPV in dogs since puppies are more susceptible; thus, you have the "thought" of protecting puppies via vaccine

Diagnostic

1. ruled in via differential diagnosis

2. increased likelihood of disease development due to exposure

Management

promoting vaccination hence animals with vaccines have 5 times more protection than unvaccinated animals

Goals of epidemiology

1. describe health status

2. causality vs. association

3. predict disease occurrence

4. control disease spread

Did the researcher assign exposures?

1. yes: experimental study

2. no: observational study

Is the allocation random in the experimental study?

1. yes: randomized controlled trial

2. no: non-randomized controlled trial

Is there a comparison group in the observational study?

1. yes: analytical study

2. no: descriptive study

Analytical study types

1. case-control

2. cross-sectional

3. cohort

Descriptive study

1. just comparing; identifying the host and environment of the disease

2. the focus is on individual/population with "newsworthy" clinical occurrence

3. prefers unusual cases

4. more qualitative than quantitative since it only describes what happened

Descriptive study examples

1. case report

2. case series

Case report advantages

1. intensive since only focuses on 1 disease

2. detailed

Case report disadvantages

1. subjective in nature hence prone to bias

2. "focuses" on new disease

Case series

multiple and continuous unusual case occurrences

Case report examples

1. A cartilaginous choristoma in a pig liver: a case report

2. Silent histomoniasis on a brooder farm

3. CPV infection in Turkey

Cases series examples

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

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

Analytical study

there is a comparison group; comparing diseased and non-diseased populations

Assessing causation using the first three rules of Evan:

1. higher prevalence in exposed

2. higher incidence in exposed

3. exposure to a cause should be present more in a diseased population

when the animal is exposed = it has higher probability of getting diseased

Types of analytical studies

1. cohort

2. case control

3. cross sectional

Cohort study

1. most effective in assessing causal hypotheses

2. example: investigate the effect of spaying on urinary incontinence

3. starts with the cause before getting the effect

4. studies the progression of the exposed group

2 types of cohort study

prospective and retrospective

Cohort study starts with a population that has a?

similar characteristic

Cohort study answers the question?

"will the disease develop of not?"

Cohort study groups are divided into?

exposed and unexposed

Prospective cohort study

start with today and check the outcome tomorrow

Retrospective cohort study

start with yesterday and check the outcome today

Cohort study diagram

example of a cohort study: starts with the population that drinks colostrum from infected mothers

exposed: intact female dogs, unexposed: spayed dogs

Cohort

a population with a shared characteristic (e.g. students taking VPH 121)

Cohort study advantages

1. calculation of incidence (or cumulative incidence aka "risk" since there is development of new cases in cohort studies)

2. flexible in choosing variables (no control over who is getting exposed hence, there is no bias)

3. investigate multiple outcomes and potential risk factors

4. rare exposures (since subjects are selected by their exposure status)

5. progression of the disease

6. temporal cause and effect relationship

7. second choice if experimental studies are unethical

Cohort study disadvantages

1. sampling error (since you have to select samples because you cannot sample everyone)

2. large population size for rare diseases (a small population will lead to low prevalence and incidence values)

3. long duration of follow-up

4. difficulty in follow-up

5. high cost

6. confounding variables (common when the study gets prolonged)

Sampling error in cohort studies

since subjects are selected, the sampling error can be disregarded due to overestimation or underestimation

Cohort study and cumulative incidence

1. cumulative incidence must be calculated in a cohort study

2. in a cohort study, the more appropriate measure to compute is the risk

Cohort and experimental study

similar; but there is a "control " in experimental studies

Relative risk

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

Relative risk answers the question?

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

Prospective and retrospective cohort studies diagram

Attributable risk

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

Attributable risk formula

AR = AR exposed - AR unexposed

AR > 0

excess absolute risk

AR = 0

absence of additional risk (there is still a risk but there is no additional)

AR < 0

not a risk factor, protective factor

Case control study selects?

diseased and disease-free populations

Case control study is effective in studying diseases with?

low incidence and conditions with long follow-up

Case control study has no measure of disease frequency because?

sampling is chosen due to disease presence

Case control study is the opposite of cohort study since?

we will start with the outcome and check previous exposures

Prospective case control study

non-existent; a case control study will always be retrospective

Case control study

1. incidence and prevalence cannot be computed since there are no new cases and there is a predetermined population

2. example: investigate the risk factors for those with urinary incontinence

3. an example of a longitudinal study since there is a duration of time

4. studies diseased vs. disease-free populations

Case control study starts with?

outcome then checking the history of the exposed and unexposed groups

Case control study answers the question?

"what happened?"

Case control study advantages

1. rare diseases

2. diseases with long incubation or latent periods

3. fast conduct

4. low cost (since less time is spent and records are used instead; thus, there is no follow-up)

5. few subjects are required (looks at outcome already)

6. available records

7. absence of risk

8. multiple exposures

Case control study disadvantages

1. sampling error

2. poor quality of records

3. difficulty in validation

4. no control on variables

5. selecting a control group

6. no assessment of temporal sequence (since the effect came first before the cause; opposite of the natural course of diseases)

7. rare exposures (since the cases already happened, hard to assess exposure)

8. limited to one outcome

Case control study 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

1. random selection and examination of a population at

one point in time

2. describing disease occurrence at the time of collection

3. only type of study that is not longitudinal

4. records can be used (similar to case control studies)

A cross-sectional study can be challenging when investigating causal hypotheses, because?

there is no cause in the equation since cross-sectional study only checks the outcome prevalence

Cross-sectional study has no idea about the?

possible exposures that happened in the past

What type of study is: "How many developed urinary incontinence in dogs from Batong Malake during October 2023? What are possible risk factors?"

cross-sectional study

2 MULTIPLE CHOICE OPTIONS

Prevalence of cross-sectional studies is?

computed since there is a need for the number of diseased animals at a certain time

Cross-sectional study advantages

1. estimation of prevalence (in larger populations) or positivity rate (in smaller populations)

2. fast conduct

3. moderate cost (since it only identifies animals with the disease)

4. records can be used occasionally

5. no risk to subjects (except for the subject involved in sample collection)

6. multiple exposures and outcomes

Cross-sectional study disadvantages

1. rare diseases and exposures (since there will be low prevalence)

2. diseases with short duration (checking the disease at one period of time; the result would turn out with low prevalence)

3. uncontrolled extraneous variables (no idea of what happened before and after)

4. estimation of incidence (not monitored into time)

5. temporal pattern (since it can be non-established)

6. poor quality of records

Cohort study flow

cohort group -> exposure -> outcome

Case control flow

outcome -> exposure

Cross-sectional study flow

absent = the exposure and outcome happens at the same time since it is cross-sectional

Analytical study diagram

Descriptive study diagram