3: How do you describe disease occurrence?

Question 1

Why are we interested in disease within populations rather than individuals?

Answer 1

• Population = more individuals = more information
• Every individual might not be available for examination
• Occurrence in populations can inform us about individual risk
• Some things only happen at a population level

Question 2

How do we define populations broadly?

Answer 2

A group of units, elements, individuals connected by some common characteristics.

Question 3

When does the definition of a population differ?

Answer 3

It is different depending on the discipline.

Question 4

How do we define populations in epidemiology?

Answer 4

Connected by their relevance to the outcome of interest (population at risk)

Question 5

For a population definition to be useful you need to determine …

Answer 5

What, where, when?

(Characteristics, space, time)

Question 6

What are different population characteristics?

Answer 6

• Structure (contiguous or separate or meta-populations)
• Managed/unmanaged
• Dense/sparse
• Homogenous/heterogeous

Question 7

What populations are easier/harder to count?

Answer 7

Contiguous = difficult to count
Separated = easier to count

Question 8

Can we study the entire population?

Answer 8

No (in most cases)

Question 9

What is a target population?

Answer 9

Population about which we are drawing inferences.

Question 10

What is a source population?

Answer 10

Population from which the sample is drawn.

Question 11

What is a study population?

Answer 11

Group of individuals selected for the study (sample).

Question 12

Why might target and source populations be different?

Answer 12

• Access
• Bias

Question 13

What do we want in a sample?

Answer 13

To be as representative of the target as possible.

Question 14

How do we sample populations?

Answer 14

Non-probability sampling - some individuals do not have any probability to be sampled.
- Convenience
- Purposive

Probability sampling - all individuals have a chance to be sampled.
- Simple random sampling
- Systematic random sampling
- Stratified random sampling
- Clustered random sampling

Question 15

What is convenience sampling?

Answer 15

An example of non-probability sampling.

Ex. Sampling from a pasture that is closest to access.

Question 16

How do we quantify disease occurrence?

Answer 16

Ratio, proportion, and rates.

Question 17

What is a ratio?

Answer 17

A value obtained by dividing one quantity (the numerator) by another (the denominator).

Question 18

What is a proportion?

Answer 18

A ratio in which the numerator is part of the denominator (x/x+y).

Question 19

What is a rate?

Answer 19

A ratio that indicates the change in one quantity with respect to one or more others over time.

Question 20

What is prevalence?

Answer 20

The proportion of the population that has a specific disease at a given time.

Question 21

What are prevalent cases?

Answer 21

Number of cases of a disease at a given time.

Question 22

How do you calculate prevalence?

Answer 22

Number of cases / size of population

Question 23

A population has a disease prevalence of 0.13. What does that value mean?

Answer 23

There is a 13% chance that an individual in that population has that disease.

Question 24

What factors can affect a prevalence estimate?

Answer 24

• Test specificity & type of test
• Case definition
• Population definition

Question 25

How can you describe the dynamic of a disease over time?

Answer 25

- Incident cases - Incident risk

Question 26

How is incidence risk calculated?

Answer 26

The number of new cases / the number of individuals initially at risk *****Within a defined time period*****

Question 27

What is incidence risk?

Answer 27

The average risk of developing a disease in a population within a specific time period. (aka. cumulative incidence)

Question 28

What individuals are not included in an incidence risk calculation?

Answer 28

Those that are already sick before the period starts.

Question 29

How is incidence rate/incidence density calculated?

Answer 29

Number of new cases in a time period / the sum of length of time at risk of developing the disease (in all individuals)

Question 30

What value can incidence rate be?

Answer 30

Any number between 0 and infinity.

Question 31

What is the unit for incidence rate?

Answer 31

/animal-week, /animal-year, etc.

Question 32

How do you calculate the approximate incidence rate?

Answer 32

IR approx = Number of new cases / (Duration*(number at risk at the start of the time period + number at risk at the end of the time period)/2)

Question 33

When would you use the approximate incidence rate?

Answer 33

If you don’t have an exact timeline for all animals.

Question 34

When would you choose prevalence vs incidence?

Answer 34

Depends on needs and research question. Prevalence: - Focus on existing cases - Measure of disease burden - Evaluate diagnostic tests Incidence: - Study dynamic of disease - Exploring causal theories - Evaluating preventive measures

Question 35

What is an attack rate?

Answer 35

Cumulative incidence at beginning of outbreak or following short exposure.

Question 36

What is the cumulative mortality?

Answer 36

Disease-related death incidence risk (rate).

Question 37

What is case fatality?

Answer 37

Number of death/number of diseased animals

Question 38

What is death rate?

Answer 38

Deaths from all causes.

Question 39

What is infection?

Answer 39

Invasion by and multiplication of microorganisms in a bodily part or tissue, which may produce subsequent tissue injury and progress to overt disease through a variety of cellular or toxic mechanisms.

Question 40

What is disease?

Answer 40

Deviation from the normal structure or function of any body part, organ, or system that is manifested by a characteristic set of symptoms and signs and whose etiology, pathology, and prognosis may be known or unknown.

Question 41

What is the effect of sampling?

Answer 41

You have a sub-group of individuals.

Question 42

How does sampling impact the measure of occurrence?

Answer 42

It is slightly different every time / for every sample. (= random error)

Question 43

How do we express when reporting results? (this sentence doesn't make a ton of sense to me but its right from the notes)

Answer 43

Confidence interval

Question 44

What does a confidence interval mean?

Answer 44

When sampling many times and constructing CI, most (95%) of them will include the true occurrence value.

Question 45

How does confidence interval change with sample size?

Answer 45

Small sample size = large confidence interval