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Table 1 Alternative ways to describe populations.

From: PopMod: a longitudinal population model with two interacting disease states

Logical expression Meaning Differential equations expression
~ X~ C Population group with neither X nor C S
X~ C Population group with X but not C, i.e. with X only X
~ XC Population group with C but not X, i.e. with C only C
~ X Population group without X S + C
~ C Population group without C S + X
X Total population group with X X + XC
C Total population group with C C + XC
S Susceptible population S
XC Population with both X and C XC
T Total population T