I recent stumbled upon a formula for Prevalence ratio which is
(observed prevalence)/(Expected prevalence)
with Observed prevalence is
(Total number of cases with disease A and B)/(Total number of cases)
and Expected Prevalence is
(Observed Prevalence of Disease A)*(Observed Prevalence of Disease B).
Till now I'm used to 2 by 2 contingency table with formula as p1/p2, where
p1 = a/a+c and
p2 = b/b+d
I would like to know the difference between these two approach and appropriate places to use them.