measures of disease frequency

Question 1

what is the importance of quantifying the level of disease in a population?

Answer 1

quantifying diseases frequency helps to:

• assess impact & importance of disease in populations
• plan, implement & evaluate disease control programs
• monitor trends of disease over time

Question 2

what are 3 common measures of disease frequency

Answer 2

1. proportion - a proportion is a fraction in which the numerator is included in the denominator
2. ratio - expresses the relative size of 2 independent numbers
3. rate - quantifies the relationship between an event & a defined-population at risk evaluated over a specific time

Question 3

define morbidity

Answer 3

• refers to individuals who are sick or unwell

- used to refer to extent of disease or disease frequency within a defined population

Question 4

closed population vs open population

Answer 4

closed population
- refers to a population with no additions (no animals being added to the study) & few or no losses during the study period

open population
- refers to a population where animals may be entering or leaving throughout the study period

Question 5

what is prevalence?

Answer 5

• is the proportion of sampled individuals with a defined condition of interest (infectious disease or other defined characteristic) at a point in time
• does not distinguish between old or new cases
• numerator includes all animals that are cases at time of study regardless of when illness began
• prevalence = number of exisiting cases/size of population

Question 6

what is incidence?

Answer 6

• measures the number of new cases that arise in a population over time

Question 7

what is incidence risk?

Answer 7

• is a measure of disease occurrence over a period of time

- it is a proportion, so it takes values from 0 to 1 (0% to 100%)

Question 8

define incidence rate?

Answer 8

• incidence rate takes into account the time an individual is at risk of disease
• it is not a proportion since it defines the number of cases per animal-time at risk
• describes how quickly disease occurs in a population

Question 9

how do you know when to measure incidence risk or rate?

Answer 9

• for short study & risk periods/closed population -> measure RISK
• for long study & risk periods/open population -> measure RATE

Question 10

what is case fatality risk?

Answer 10

• is the proportion of animals with a specific disease that die from it within a specific time period
• reflects the prognosis among animals with the disease

Question 11

what is attack risk?

Answer 11

• is a specific type of cumulative incidence (incidence risk) which applies to outbreaks or situations where the period of observation is relatively short