fraction where numerator is NOT part of denominator examples: 6.1 ticks : 1 sheep diseased : non diseased remember ratios use : NOT /

Measures of Disease Occurrence Flashcards by Bryan Greenberg

What is epidemiology concerned with?

**presence **of existing health problems

occurence of new health events in population

*** quantification **of there measurments

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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

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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

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When measuring disease occurene it is important to define?

what

who (Study population)

location / place

time period of study

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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

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Types of Measure?

Count

Proportion

Rate

Ratio

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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

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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

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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 )

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What is the meaning of risk factor?

determinant of disease

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what is the most common rates?

cumulative incidence

incidence density

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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 /

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What is the most common proportion?

** prevalence**

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Types of prevalence?

Types of incidence?

**Point prevalence **

Period prevalence

Cumulative incidence (attack rate)

Incidence density or Incidence rate

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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

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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

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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

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