Physician-researchers at a women's health clinic that caters to an underserved population are reviewing the results of a study they conducted regarding the effect of support systems during and after pregnancy on the development of postpartum depression.
Based on the results of the study, what was the approximate incidence of postpartum depression regardless of type of support system?

Question

Physician-researchers at a women's health clinic that caters to an underserved population are reviewing the results of a study they conducted regarding the effect of support systems during and after pregnancy on the development of postpartum depression. 
Based on the results of the study, what was the approximate incidence of postpartum depression regardless of type of support system?

Quizplus · Accepted Answer

Explanation: This cohort study evaluated the association between support systems during and after pregnancy and the development of postpartum depression (PPD). One cohort consisted of 1,500 women in their first trimester of pregnancy who had a good support system, and a second cohort consisted of 3,500 women in their first trimester of pregnancy who had a poor support system. Both cohorts were followed for 24 months to estimate the incidence of PPD, and the outcomes were as follows: Among 1,500 women with a good support system: incidence of PPD = 6% (or 0.06) Among 3,500 women with a poor support system: incidence of PPD = 36% (or 0.36) Because the cohorts had different sample sizes, the overall incidence of PPD (ie, incidence regardless of type of support system) must be calculated using a weighted average of the incidence in both cohorts. The weighted average is calculated by multiplying the incidence in each cohort by its weight (ie, the proportion of participants in each cohort) and then adding the results. Weighted average = (Incidence in cohort 1 × Cohort 1 weight) + (Incidence in cohort 2 × Cohort 2 weight) The total number of participants in the study is 1,500 + 3,500 = 5,000, so the weight for the cohort of women with a good support system is 1,500 / 5,000 = 0.30. The weight for the cohort of women with a poor support system is 1 − 0.30 = 0.70. Weighted average = (0.06 × 0.30) + (0.36 × 0.70) = 0.27 (or 27%) Therefore, the approximate incidence of PPD regardless of type of support system is 27%. (Choices A and D) The question asks for incidence of PPD regardless of type of support system; 6% and 36% are the incidence of PPD among women with a good support system and among women with a poor support system, respectively. (Choice B) The simple average incidence of PPD between cohorts is 21% [(0.06 + 0.36) / 2 = 0.21]. However, because the cohorts had different sample sizes, calculating the weighted average of the incidence of PPD between cohorts is necessary to determine overall incidence. Educational objective: Incidence refers to the proportion or rate of persons who develop a condition during a particular period.

Physician-Researchers at a Women's Health Clinic That Caters to an Underserved