Definitions Flashcards
The absolute number of animals that have a particular disease…
Count
The count of diseased animals are expressed as a fraction of the animals that could be diseased…
proportion
The most common proportion is…
prevalence
An expression of the change in the count or amount of disease in a defined population per unit of time…
rate
Most common rates are…(2)
cumulative incidence and incidence density
A fraction in which the numerator is NOT part of the denominator…
ratio
The ratio of the number of diseased to the number NOT diseased…
Odds ratio
The proportion of the study population that is diseased at any one time…or the measure of the amount of disease in the population at any one time…
prevalence
The proportion of the study population that is diseased at a single point in time…or the probability that an individual in the study pop. is diseased at the time of study…
point prevalence
How do you calculate point prevalence?
cases of disease present in the pop. at a particular time/ total pop. at that specified time
The proportion of the study pop. that is diseased during a specified period of time that measures all the cases of disease in the study pop. over a period of time is…
period prevalence
What type of prevalence includes old cases of disease that were present at the beginning of the time period AND new cases of disease that occur over the time period?
period prevalence
How do you calculate period prevalence?
cases of disease present in the pop. over a period of time/ total pop. over that period of time
What is the number of new cases of disease that occur in the study pop. over time?
Incidence
Incidence is used to predict…
case occurrence in a population
Incidence only counts…
NEW CASES of disease
The proportion of disease-free (susceptible) individuals in a pop. who became diseased during a specified period of time…
cumulative incidence
How do you calculate cumulative incidence?
new cases of disease or events during the time period/ total disease free pop. at risk at the beginning of the time period
What is often determined by cross-sectional studies and surveys?
Point Prevalence
What is cumulative incidence during an epidemic or the probability (risk) of becoming diseased during the course of an epidemic?
Attack Rate
How do you calculate attack rate?
new cases of disease during an epidemic/ total pop. at risk or exposed during that epidemic
What is an instantaneous rate of occurrence of new cases of disease among non-diseased animals in the population?
Incidence density (incidence rate)
How do you calculate incidence density or incidence rate?
new cases of disease during the study/sum of length of time during which each individual in the pop. is at risk
How do you report incidence density?
reported as animal-time at risk
How do you report period prevalence?
cases/animal/time
How do you report cumulative incidence?
cases/animal/time
What measures the amount of disease in a population?
prevalence
What measures the rate of disease occurrence?
incidence
What is the best measure to assess the overall burden of the disorder, including the costs and resources and reflects the overall magnitude of the disease?
prevalence
What is the best measure to asses increasing or decreasing trends in disease frequency and is used to asses the risk factors/causes for the disease?
incidence
What is defined as the probability of becoming disease?
Disease risk
When an individual is not diseased but is capable of becoming diseased, we say these individuals are…
at risk
What are factors that increase the probability of becoming diseased?
risk factors
What is the incidence of death in a given time period?
Mortality rate
How do you calculate Mortality Rates?
deaths during the time period/ total pop. during the same time
What is the mortality rate due to a specific disease or event?
cause-specific mortality rate
How do you calculate cause-specific mortality rates?
deaths from a specific cause or incident/ total pop. during the same time
What is defined as the # of cases that are fatal, within a specified time, following disease onset or diagnosis?
Case Fatality Rate
How do you calculate case-fatality rate?
individual cases of disease that die/total #individuals with the disease
In enzootic disease, transmission is sustained over time due to…
- the result of regularly occurring exposures
2. a continuous supply of susceptible animals
What is a concept in nature of a pop.-level phenomenon which accounts for the reduction in transmission of infectious disease?
herd immunity
What are the sources of immunity?
innate, passive or acquired
Highly infectious diseases require what amount of immunity?
approximately 95% immunity
How can you measure herd immunity?
study the prevalence of antibodies in a population
What is the number of secondary cases caused by an infected individual in an entirely susceptible population that determines whether a disease can persist and is valuable for assessing management options?
R0
What is a measure of the basic reproductive rate, transmissibility, or spreading potential of an infection in a population?
R0
How is R0 calculated?
(#contacts per day) X (probability of transmission per contact) X (duration of infectivity)
Diseases are controlled when the R0 is…
How do you approximate what % of the pop. that needs to be immune to prevent or stop disease transmission by manipulating R0?
Multiply R0 by the proportion of the pop. that is NOT immune –> R0 X (%naive pop.)
If this reduces R
What is based upon the probability of transmission from a population of infected cases to a population of susceptible animals?
Reed-Frost Model
The Reed-Frost Model is an iterative model that provides a “snapshot” in time, allowing predictions of entire epidemic curves and accounts for the effects of immunity by…
removing immune animals from the “S” or susceptible population with each iteration.
The Reed-Frost Model is good when dealing with what type of population?
Large, closed populations
Goals of epidemiological studies are…
- describe the disease (descriptive studies)
2. identify associations (analytical studies)
A good study should be…
- Scientifically sound
- Valid
- Precise
- Efficient
Most Epidemiological studies are…observational or experimental?
Observational
When researchers do not know the treatment or exposure status of the subjects, this is called…
blinding
What type of study is Ecologic Fallacy a disadvantage and what does that mean?
Ecologic Fallacy is found in Ecological (Analytical) studies.
Ecologic Fallacy = the relationship in the pop. may not be true at an individual level
What is a tool or method for collecting information about individuals in populations? What studies commonly utilize this method?
Surveys
Cross-Sectional Studies commonly utilize surveys.
