If we use the below table for presenting data of a cohort study, the incidence (risk) of the outcome in the unexposed group can be presented by the formula:

	Outcome
Exposure	Yes	No	Total
Yes	a	b	a+b
No	c	d	c+d
Total	a+c	b+d	a+b+c+d

a. a/(a+b)

b. c/(c+d)

c. b/(a+b)

d. d/(c+d)