ABSTRACT
The traditional methods of studying the gene-environment interactions need a control group. However, the selection of an appropriate control group has been associated with problems. Therefore, new methods, such as case-only design, have been created to study such interactions. The objective of this study was to compare the case-only and case-control designs using data from patients with breast cancer. The interaction of genetic and environmental factor as well as the ratio of control to population odds ratio was calculated for case-only [300 patients with breast cancer] and case-control [300 cases of breast cancer and 300 matched controls] designs. The confidence intervals and -2log likelihood in all variables in case-only design was smaller than those in the matched case-control design. In case-only design, the standard errors of some variables such as age at menarche, the first delivery at the age of 35 yrs and more or no delivery, the history of having live birth, use of oral contraception pills, breastfeeding history were less than those in the matched case-control design. The findings indicate that the case-only design is an efficient method to investigate the interaction of genetic and environmental factors