The design is a 2x2 factorial design and we will power the study to detect a difference in proportions between the four groups. If we assume a power of 80% and an alpha error of 5% we would require 67 patients in each group to detect a difference of proportions of 17%, 20%, and 35% in the groups that received either intervention or both combined. We assumed that the combination of patient and physician letter would lead to higher screening rates. A larger sample size is required when an interaction is suspected to ensure that the study has sufficient power to detect an interaction.16 The total sample size of the study would be 268 patients.