Measures of dz Flashcards
What is the most commonly-used proportion in epidemiology?
Prevalence
What type of measurement includes a fraction in which the numerator is NOT part of the denominator?
Ratio
What is a commonly-used ratio in epidemiology?
“The odds”
Cases: non cases
(# cases for every # non-cases)
What is a study population?
The “n” or “sample” being handled/tested
What is a source population?
The population from which the study is related to…(stray cats in Basseterre)
What is the target population?
The population that we may generalize the pattern of results
(All cats on St Kitts)
Measures of morbidity
Prevalence
Incidence
How is prevalence measured?
Proportion of the study population that is diseased at any one time;
The measure of amount of disease in the population at any one time;
Interpret as the probability of an animal from the study population BEING DISEASED
can provide information about how frequently you might expect to see the conditions in your practice
What is point prevalence?
The proportion of study population that is diseased at a SINGLE point in time
What is the most common measure of prevalence?
Point prevalence
How do we calculate point prevalence?
cases of disease present in the population at a particular time/ total population at that time
Can be expressed as a percentage, proportion, or fraction
What is period prevalence?
Proportion of population that is diseased during a specified period of time;
Measures ALL the cases of disease in the study population over a period of time;
Includes:
Old cases, new cases, but does not tell us WHEN the animals became diseased…
How do we calculate period prevalence?
cases of disease present in the population (OVER A PERIOD OF TIME) / total population over that period of time
What is incidence?
The number of NEW cases of disease that occur in the study population over time;
The probability of a disease-free animal from the study population becoming diseased…
AKA THE RISK OF BECOMING DZ’d
Cumulative incidence is ?
Proportion of disease-free (susceptible) individuals in the study population who became diseased during a specified period of time
*can be interpreted as the probability of becoming Dz’d during the study time period
How do we cumulative calculate incidence?
NEW cases of disease in a population over a specified time period / total population at RISK at the start time of the time period
How can cumulative incidence be expressed?
- a number (with reference to time and population at risk)
- a dimensionless fraction with reference to time
What are the assumptions that come with cumulative incidence?
- All individuals in the study population are at risk of getting disease
- Every individual in study population is assessed at the start of the study, to identify and remove EXISTING cases
- The ENTIRE population at risk must be followed from the start of the study until the end
What is attack rate?
A specific type of cumulative incidence;
Cumulative incidence DURING an outbreak
Applied to a narrowly-defined population, observed for a limited period of time
How can we interpret attack rate incidence?
The probability (risk) of becoming diseased DURING the course of an outbreak
How do we calculate attack rate?
new cases in a population over a specified time period / total population at risk (exposed) at the start of the time period
What is the incidence rate?
The instantaneous rate of occurrence of new cases of disease among NON-diseased animals in the population
aka incidence density
What is the drawback of incidence rate?
Cannot easily interpret incidence rate at the INDIVIDUAL animal level
How do we calculate incidence rate?
new cases of disease in a population over a specified time period / SUM of the length of time during which each individual in the population is at risk
