# Prevalence Ratio & Prevalence Difference

Suppose we had data from a cross-sectional survey that had asked whether people currently smoked and whether they currently had problems with wheezing and coughing. You could use a 2x2 table to organize the data from the 100 respondents as in the table below:

Wheeze/Cough (yes)

Wheeze/Cough (no)

Total

Smokers

13

13

26

Non-smokers

2

72

74

• Prevalence of wheezing and coughing among smokers = 13/26=0.50.
• Prevalence of wheezing and coughing among non-smokers = 2/74=0.027.

Prevalence Ratio = PR = 0.50/0.027 = 18

Interpretation: Smokers had 18 times the prevalence of wheezing and coughing compared to non-smokers.

Prevalence Difference = PD = 0.5-0.027=0.473 = 47.3 per 100

Interpretation: Among smokers there were 47 excess cases of wheezing and coughing per 100 compared to non-smokers in a given time period.

Based on this cross-sectional data, I might hypothesize an association between smoking and respiratory problems, but the temporal relationship is unclear, and an analytical study would be a good next step.