Measures of Association in Epidemiology

Question 1

what is a count

Answer 1

= a number of something (e.g. infected animals, disease cases)

Question 2

what is a proportion

Answer 2

= A ratio in which the numerator is a subset of the denominator. (e.g., prevalence, incidence risk)

Question 3

what is a rate

Answer 3

= A ratio in which the denominator is the number of animal-time units at risk (e.g., incidence rate, mortality rate)

Question 4

what is odds

Answer 4

= A ratio in which the numerator is not a subset of the denominator

Question 5

Prevalence (about)

Answer 5

= A type of proportion (A ratio in which the numerator is a subset of the denominator)
- Cases existing at specific point in time (point prevalence) or during a specified period of time (period prevalence).
- It can be viewed as a snapshot of the disease situation in a population at a point in time
- Takes on values from 0 to 1 (i.e. 0% to 100%)
P=
Number of cases of disease in a population at a given time
__________________
Number of animals in the population at that time

Question 6

Prevalence (example)

Answer 6

• A group of cows is tested for EBL.
• 40 are positive and the remaining 160 are negative.
• 40/(40+160)
  = 40/200
  = 0.2

Question 7

Incidence risk (about)

Answer 7

• Another example of a proportion
• Takes on values from 0 to 1 (i.e. 0% to 100%)
• R= number of newly affected individuals in a defined time period
  __________________
  population at risk (i.e. # disease free at the beginning of time period)

Question 8

Incidence risk (example)

Answer 8

102 healthy dogs are monitored over the course of a week. There are 4 new cases of kennel cough in these dogs during this time period.
R= 4/102 = 0.039 (i.e. 3.9%)

Question 9

What are the two different types of incidence calculations

Answer 9

• incidence risk

- incidence rate

Question 10

Incidence rate (calculation)

Answer 10

• example of a rate
  I= number of cases of disease in a defined time period
  ______________________
  number of animal-time units at risk during the time period

Question 11

Incidence rate (example)

Answer 11

30 cases of kennel cough in a 100-dog kennel over 3 mo. Incidence rate: 30/(100*3) = 0.01 cases per dog-month
(denominator is 300 dog months

Question 12

Odds (example)

Answer 12

= number of cases/ number of non-cases
- A group of cows is tested for EBL. 40 are positive and the remaining 160 are negative.
40/160 = 0.25 or 1:4

Question 13

what are measures of association and 3 main types

Answer 13

• Used to evaluate the magnitude of the relationship between the disease and the exposure
• 3 main types:
  ~ The risk ratio (RR) also called relative risk
  ~ The odds ratio (OR)
  ~ The incidence rate ratio (IR)
  ABOUT EACH OF THEM IS PRINTED AND IN FOLDER

Question 14

Limitations of risk ratio

Answer 14

• doesn’t tell us about how much disease is occurring in the population
• The actual disease frequency can be quite low, but the RR can still be very high.
• E.g., the risk of eye cancer in horses with white eyelids is 3.8 times higher than that of horses with pigmented lids
• E.g., your risk of dying from a vending machine falling on you is 6 times higher than dying from a shark attack (but both are extremely rare)

Question 15

Limitations of incidence rate ratio

Answer 15

• The ratio of disease frequency (measured as incidence rates) in an exposed group to the incidence rate in a non-exposed group.
• IR can only be calculated from certain studies
  ~ IR cannot be calculated in cross-sectional studies because we don’t know how many cases of the disease are “new”
  ~ In case-control studies, you are selecting based upon disease so IR doesn’t make sense (you select a certain number of infected and uninfected individuals)
  ~ IR is useful in cohort studies

Question 16

What measures are appropriate for specific study design

Answer 16

- Cross sectional 
~ risk ratio and odds ratio
- Cohort study
~ risk, incidence and odds ratio
- Case-control
~ odds ratio