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Epidemiology > 3: How do you describe disease occurrence? > Flashcards

3: How do you describe disease occurrence? Flashcards

1
Q

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

A
  • 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
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2
Q

How do we define populations broadly?

A

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

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3
Q

When does the definition of a population differ?

A

It is different depending on the discipline.

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4
Q

How do we define populations in epidemiology?

A

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

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5
Q

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

A

What, where, when?

(Characteristics, space, time)

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6
Q

What are different population characteristics?

A
  • Structure (contiguous or separate or meta-populations)
  • Managed/unmanaged
  • Dense/sparse
  • Homogenous/heterogeous
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7
Q

What populations are easier/harder to count?

A

Contiguous = difficult to count
Separated = easier to count

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8
Q

Can we study the entire population?

A

No (in most cases)

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9
Q

What is a target population?

A

Population about which we are drawing inferences.

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10
Q

What is a source population?

A

Population from which the sample is drawn.

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11
Q

What is a study population?

A

Group of individuals selected for the study (sample).

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12
Q

Why might target and source populations be different?

A
  • Access
  • Bias
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13
Q

What do we want in a sample?

A

To be as representative of the target as possible.

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14
Q

How do we sample populations?

A

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

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15
Q

What is convenience sampling?

A

An example of non-probability sampling.

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

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16
Q

How do we quantify disease occurrence?

A

Ratio, proportion, and rates.

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17
Q

What is a ratio?

A

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

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18
Q

What is a proportion?

A

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

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19
Q

What is a rate?

A

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

20
Q

What is prevalence?

A

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

21
Q

What are prevalent cases?

A

Number of cases of a disease at a given time.

22
Q

How do you calculate prevalence?

A

Number of cases / size of population

23
Q

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

A

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

24
Q

What factors can affect a prevalence estimate?

A
  • Test specificity & type of test
  • Case definition
  • Population definition
25
Q

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

A
  • Incident cases
  • Incident risk
26
Q

How is incidence risk calculated?

A

The number of new cases / the number of individuals initially at risk

Within a defined time period

27
Q

What is incidence risk?

A

The average risk of developing a disease in a population within a specific time period.

(aka. cumulative incidence)

28
Q

What individuals are not included in an incidence risk calculation?

A

Those that are already sick before the period starts.

29
Q

How is incidence rate/incidence density calculated?

A

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

30
Q

What value can incidence rate be?

A

Any number between 0 and infinity.

31
Q

What is the unit for incidence rate?

A

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

32
Q

How do you calculate the approximate incidence rate?

A

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)

33
Q

When would you use the approximate incidence rate?

A

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

34
Q

When would you choose prevalence vs incidence?

A

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

35
Q

What is an attack rate?

A

Cumulative incidence at beginning of outbreak or following short exposure.

36
Q

What is the cumulative mortality?

A

Disease-related death incidence risk (rate).

37
Q

What is case fatality?

A

Number of death/number of diseased animals

38
Q

What is death rate?

A

Deaths from all causes.

39
Q

What is infection?

A

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.

40
Q

What is disease?

A

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.

41
Q

What is the effect of sampling?

A

You have a sub-group of individuals.

42
Q

How does sampling impact the measure of occurrence?

A

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

43
Q

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

A

Confidence interval

44
Q

What does a confidence interval mean?

A

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

45
Q

How does confidence interval change with sample size?

A

Small sample size = large confidence interval

Epidemiology (9 decks)

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