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Formula:

- If data is available at two time points (base and end of the short term,
usually a year), then use the following formula:

P 0 + P 1
P tb =
2

Where:

P tb: Average population;

P 0: Population at the base period;
P 1: Population at the ending period.

- If data is available at times evenly, then use the formula:

P 0 P n
+ P 1 + .... + P n-1 +
P tb = 2 2
n

Where :

P tb: Average population;
P 0,1,...,n: Population at time points of 0, 1,...,;

n : Number of equal time points.

- If data are available at times unequal spaces, using the formula:

P tb1t 1 + P tb2t 2 + .... + P tbnt n
P tb =
∑t i

Where:
P tb: Average population;

P tb1: Average population of the first duration;

P tb2: Average population of the second duration;
th
P tbn: Average population of the n duration;
th
t i: Length of the i duration.

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