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
How is incidence rate expressed?
cases / animal-time at risk
Ex) 15 cases per 100 cow-months at risk
Anything that affects the ______ of disease will affect prevalence
Duration
Anything that affects the _______ can influence the prevalence.
Change in # of new cases
What are some benefits of prevalence over incidence?
Reflects the overall magnitude of a disease; is the best measure to assess the overall burden of the disorder (including costs and resources)
What is the case-fatality rate?
of cases of a specific disease that are fatal, within a specified time following disease onset
How is case-fatality rate calculated?
cases of disease that die in a specified time period following disease onset / total number of individuals with the disease
When measuring disease occurrence it is important to clearly define what 4 things?
- what is being measured (dz, infection, etc)
- The persons or animals included in the measurement
- Place or location of the study population
- Time period of the study
What are the 4 most common types of measures in epidemiology?
- counts
- proportions
- ratios
- rates
All other measures of disease occurrence are based on _____, but on their own, they convey little information
Counts
The count of animals with the disease (being measured), as a fraction of the total animals that COULD be diseased is known as the __________
Proportion
In a proportion, animals in the numerator are included in the denominator. These are often expressed at fractions or percentages
T/F: The prevalence of a dz may increase when incidence remains stable but survival cases improve
TRUE
Since prevalence includes all cases - new and old. When older cases live longer = increase prevalence
Incidence is generally used for more ____ diseases
acute
- used to assess causes for dz
- best measure to assess increasing or decreasing trends in dz frequency
What is a cause-specific mortality rate? How is it calculated?
The mortality rate due to a specific disease or event
of deaths from a specific dz/even during a specified time period / Total population during that same time period
