1. In the same study of risk factors described above, the investigators also were interested in whether age at menarche (first menstruation) was associated with ovarian cancer.All 391 cases and 640 controls provided information about age at menarche, and of those, 72% of cases and 413 controls were actually at least 12 years of age at menarche.

Exposure	Ovarian cancer	No ovarian cancer
Exposed	0.72*136= 97.92	413
Non-Exposed	(391-97.92 = 293.08)	(640-413 = 227 )
	391	640

True OR = (97.92*227)/ (413*293.08) =

22,227.84 / 121,042.04 = 0.18 or 0.2

1. What is the "true" OR describing the association between being at least 12 years of age at menarche compared with being <12 and ovarian cancer?

1. Among women whose actual age at menarche was younger than 12 years, 20% reported that they were 12 or older at menarche. Given this information, what would be the observed odds ratio of ovarian cancer associated with being at least 12 years old at menarche?

1. What kind of bias is present in 2b?