Measures of Disease Occurrence Flashcards
What is epidemiology concerned with?
**presence **of existing health problems
occurence of new health events in population
*** quantification **of there measurments
What is meant by measuring disease?
in a population , group, subgroup
amount of disease at one point / period of time
changes in amount of disease over time
why measure disease?
1. to know if disease is present in a population and how it is changing in that population
**2. **Compare disease in groups
When measuring disease occurene it is important to define?
what
who (Study population)
location / place
time period of study
Define **study, source, **and target populations?
- Subjects in study
sample from target population)
- population subjects were drawn from
- populations to which results are extrapolated to
Types of Measure?
Count
Proportion
Rate
Ratio
Define **count **and why it is important to disease occurence?
**absolute number of animals with particular disease **
all other measures of disease occurance are based on counts
What is the measurement of proportion?
is the count of diseased expressed as a fraction/ percentage of the animals that could be diseased
* most common proportion is prevalence
(# dz) / (study population total) = proportion
What is Rate?
expression of change in amount of disease in a defined population per unit of time
new occurence / study population within a time period
Example:
occurence of influenze in 1 year was 3.1 cases per 1000 pigs in the study population
( 3.1 cases / 1000 pigs / year )
What is the meaning of risk factor?
determinant of disease
what is the most common rates?
cumulative incidence
incidence density
What is a ratio?
fraction where numerator is NOT part of denominator
examples:
- 6.1 ticks : 1 sheep
- # diseased : # non diseased
remember ratios use : NOT /
What is the most common proportion?
** prevalence**
Types of prevalence?
Types of incidence?
**Point prevalence **
Period prevalence
Cumulative incidence (attack rate)
Incidence density or Incidence rate
What is the concept of prevalence and what does it interpret ?
**proportion of study population that is diseased at any one time or specified period of time **
**interprets: **probability of being diagnosed
how frequently to expect condition in practice
What is point Prevalence and what does it interpret?
proportion of study population that is diseased at single point in time?
**most common measure of prevalence **
interprets: probability of individual in study population is **diseased **at the time of study
What is period prevalence and what does it interpret?
proportion of study population that is diseased during specififed period of time
*** includes all cases : **old and new
**interprets: **probability that individual is diseased during period of study
What is the concept of incidence and what does it interpret?
number of new cases of disease that occur in the study population over time
**rate of occurence **
**interprets: **probability of non diseased inidividual becoming diseased ( risk of becoming dz)
The proportion of disease free (susceptible) individuals in population who became diseased during specific period of time?
**Cumulative incidence **
What does Cumulative incidence interpret and who does it include?
the probability (risk) of susceptible individual becoming diseased during study period
include only susceptible dz free at beginning
include only new cases of dz over study period
calculate cumulative incidence?
What assumptions do they give us ?
New cases of dz / total population at risk
-both during study time period
All individuals in study population are at risk
all individuals assessed at beginning
- ID and remove old cases
Must follow enitre population from start to finish
What is the cumulative incidence in this example?
Producer in St. Kits imports 150 cows from babesia free country. 12 months later, entire herd was tested and 15 cows + for babesia.
15 cases per 150 cows
or
10 cases per 100 cows
( 10 % per year )
the risk (probability) of a cow in this herd becoming infected is 10% per year
What is the term for a cumulative incidence during an epidemic and what does it interpret?
**Attack rate **
probability of becoming dz during course of epidemic
Calculation for attack rate?
new cases / total population exposed
- both during epidemic
Instantaneous rate of occurence of new cases of disease among non-diseased animals in population?
**Incidence Density or ** incidence rate
**T/F: **interpretation of incidence density (incidence rate) does **NOT **occur at any individual animal level
**TRUE **
Calculation for incidence density?
**# new cases of disease during study **
————divided————
**sum of length of time during each individual in population is at risk **
expressed as
cases per animal - time at risk
15 cases per 100 cow - months at risk
**T/F: **Only incidence density is reported as animal - time at risk?
TRUE
**T/F: **Period prevalence and cumulative incidence are reported as cases/ animals/ time?
TRUE
Prevalence vs Incidence ?
Prevalence : measure amount of dz in population
- all cases includes relative to entire population
- used to estimate probability that animal is dz
Incidence:** measure of rate of disease **
- only new cases included relative to population at risk
- there is always a time component
- Estimate # of cases of dz expected to occur
T/F: new case (incidence) can become an old case (prevalent) and remain so til death/ recovery.
- The longer old cases are dz and alive = high prevalence
- Prevalence of a disease may increase when incidence is stable but survivial of cases improves
TRUE TRUE AND TRUE
What increases prevalence?
longer dz duration
prolonged without cure
increase new cases
in migrated cases
out migrated healthy animals
imporoved diagnosis
What decreased prevalence?
short dz duration
high case fatality rate of dz
decrease new cases
in - migrate healthy
out - migrate of cases
improve cure rate
Prevalence:
is used for permanent states or chronic conditions
reflects over magnitude of dz
best measure of over burden (cost, resources, …)
TRUE
Incidence :
- used for acute aqcuired dz
- assess causes (risk factors)
- best measure to assess inc / dcr in dz frequency
TRUE
probability of becoming diseased?
Disease Risk
use incidence as risk measure
cumulative incidence = incidence risk
Individual not diseased and is capable of becoming diseased?
At risk
Risk factors
increase rhe probability of becoming diseased
Exposure to risk factor?
individual has come into contact with risk factor or has risk factor before becoming diseased
Prevalence of death in a given time period
Mortality rate
of deaths during time/ total population
Cause- Speciffic mortality rate
mortality rate due to a specific dz or event
of death from dz / total population during same time
Case fatality rate
proportion of cases that are fatal, within a specfied time following disease onset or diagnosis
of individual cases of dz that die / total # with dz
