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# 1.3.3.1 Total colony loss (TL) (the cumulative incidence of mortality)

This is the percentage of colonies lost in a specific group over a fixed period of time. This figure is the most accurate snap shot of loss in a defined group, such as in an operation or geographic region. If all colonies in a region were enumerated it would give a precise figure  for the proportion of all colonies that died in that region. However, within the population of interest, operations with large numbers of colonies will have a greater influence on the total colony loss metric than will the operations with only few colonies. Total Colony Loss in  an operation or in a defined group is calculated by dividing the total number of colonies that died over a given time period (Tdead) by the total number of colonies at risk of dying in a given time period   (TColonies at risk) and multiplying the quotient by 100% (Equation 1.3.3.1).

Where the total number of colonies at risk of dying (TColonies at risk of dying) over a period was calculated by adding the number of colonies at the start of the period (TStart) with the number of splits made by the beekeepers over the period (TSplits) and the number of colonies purchased over the period (TPurchased) and then subtracting the number of colonies removed (sold or given away) over the period (TRemoved).

And where the total number of colonies that died (TDead) was calculated by subtracting the total number of colonies at the end of a period (TEnd) from the total number of colonies at risk of dying for the period (Tcolonies at risk of dying).

Where period was the defined period of time for which colony loss was analysed. The unit of time, is the period defined by the time between TStart and TEnd. This unit is often not reported and is often loosely defined by the season encompassed by that time period (e.g. winter).

And where, respondents in a specific group are the group of respondents for whom valid loss data was collected.