How this calculator works
The inventory identity separates deaths from live removals. Period mortality here means deaths divided by opening stock plus live entries, rather than an animal-time incidence rate. Births and purchases enter the denominator; dates within the period are not modelled. Case fatality uses affected animals as its denominator and is omitted when there are no cases. Cumulative mode multiplies survival through successive conditional intervals of the same cohort, with no replacement animals.
Formula
Available = opening + births + purchases. Deaths = available - live removals - closing. Period mortality = deaths/available. Case fatality = disease deaths/cases. Cumulative cohort mortality = 1 - product(1 - interval mortality).
Worked example
Opening with 100 animals, adding 20 births, selling 10 and closing with 105 implies 5 deaths. Period mortality is 5/120 = 4.1667%. Five deaths among 20 disease cases give 25% case fatality.
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Frequently asked questions
Is this a rate per animal-year?
No. It is a period fraction using the stated inventory denominator. Animal-time rates need time at risk for each animal or group.
Why is case fatality different from mortality?
Mortality uses the population denominator. Case fatality uses only animals affected by the specified disease.
Can I add interval mortality percentages?
For conditional risks in the same surviving cohort, combine survival probabilities. For example 10% then 20% gives 28% cumulative mortality.