Lecture 1 - intro & measuring disease Flashcards
define epidemiology
the investigation of determinants of disease distribution in animal populations by using a range of different methods
explain measures of disease frequency
allow us to describe how common an illness is with reference to size of the pop. & to quantify cases with respect to time
list the 5 measures of disease frequency
- counts
- percentages
- proportions
- ratios
- rates
define ‘count’ as a measure of disease frequency. where is it applicable? what are its limits?
number of cases in a given population. ‘counts’ can be applied to infected, diseased or dead animals and may be useful in estimating workload, cost, or size of facilities to provide health care. ‘counts’ are of limited value to epi research as there is no reference. ie. size of the population is unknown. eg. 3 cases/month vs 3 cases/year
define ‘proportion’ as a measure of disease frequency. where is it applicable? what are its limits?
number/ratio in which numerator is a subset of denominator ie. a/(a+b) -> percentage if expressed per 100. it is the ‘count’ over population. can be used to find number of ‘sick’ animals vs ‘non-sick’ animals or animals ‘at risk’. does not account for time
what are 2 well known proportions in epi.?
proportional mortality =
number of deaths from disease X in year Y/total deaths
from all causes in Y
case fatality =
number of deaths due to disease X/number of new cases of disease X
define ‘ratio’ as a measure of disease frequency. where is it applicable? what are its limits?
ratios compare one quantity to another. eg. live births/fetal deaths. ie. numerator is NOT a subset of the denominator thus numbers range from 0 - infinity and cannot be expressed as a %.
define ‘rate’ as a measure of disease frequency. where is it applicable?
rates are ratios that express the change in one type of quantity relative to change in another kind of quantity. usually incorporate the concept of time or change over time. rates considered the hallmark for epi. as they form the basis of comparisons between population groups.
List 3 common forms of rates. eg.?
- crude rates: eg. crude birth rate, crude death rate
- specific rates: eg. sex-specific, age-specific
- adjusted rates: eg. age-adjusted
the 2 main measures of disease frequency in a population?
- prevalence
2. incidence
define prevalence. limitations? equation?
proportion of a population affected by a disease at a given point in time. only a measure of existing cases - includes old AND new cases
prevalence = number of prevalent cases/population present at given point in time
define incidence. what are the 2 types of incidence measures?
a measure of new cases that occur within a given time period.
2 types are RISK & RATE
Animals that can get a disease or can be diagnosed to have the disease are known as…?
population ‘at risk’
may be open or closed
define an open and closed population
open: animals enter & leave throughout the study period
closed: no additions, few to no losses during the study & only disease-free animals at start of the study considered ‘at risk’
define withdrawals
animals lost to follow up. however, population at risk can be adjusted by subtracting half the number of withdrawn animals.
define incidence risk. what are the regulations? equation? limitation?
incidence risk relates the number of new cases to the size of the population at risk in the beginning of the period studied.
it gives a probability for a disease to occur ie. the proportion of disease-free animals developing a disease over a specified time.
regulations: animals must be DISEASE-FREE at start of the time period; assumes a CLOSED population; only 1st occurrence of the same disease is counted
incidence risk = number of new cases/number of individuals initially at risk
A limitation, particularly during long study periods and specific cause of death as an outcome, is apparent when those animals are exposed to multiple or competing risks, thus difficult to obtain accurate risk estimates.
define incidence rate. equation? limitations?
incidence rate quantifies the number of new cases per animal unit of time
incidence rate = # new cases between t=0 & t=1/∑ periods ‘at risk’ of the animals
limitations include: only allow calculation of average rates over time period, thus may hide temporal patterns; total animal time does not differentiate between animals contributing shorter periods ‘at risk’ and those followed for longer periods; does not have interpretation at individual level
Explain the interpretations of incidence rate, incidence risk and prevalence.
incidence rate - number of new cases per unit of time
incidence risk - risk of developing disease over a given time period
prevalence - probability of having the disease at a particular point in time
incidence risk, rate, or prevalence? - only includes temporal sequence
incidence RISK
incidence risk, rate, or prevalence? - includes new & old cases
prevalence
incidence risk, rate, or prevalence? - includes only new cases
incidence RISK
incidence risk, rate, or prevalence? - can predict the future
incidence RISK
incidence risk, rate, or prevalence? - does not express probability or risk of disease
incidence RATE
incidence risk, rate, or prevalence? - generally used for acutely acquired diseases
incidence
incidence risk, rate, or prevalence? - useful for thinking of burden of the disorder including costs & resources consumed as a result of it
prevalence
incidence risk, rate, or prevalence? - important when thinking of aetiology of the disorder
incidence
incidence risk, rate, or prevalence? - always requires duration
incidence
incidence risk, rate, or prevalence? - animal is the unit of analysis
prevalence
incidence risk, rate, or prevalence? - the event is the unit of analysis
incidence
incidence risk, rate, or prevalence? - can never exceed 100%
prevalence
