Table 1 Alternative ways to describe populations.

~ 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