This is an observational study because the individuals were free to get married or stay single (explanatory variable). This is a cohort study.
Work Step by Step
Read the observational study definition, page 16. This is a cohort study (page 18): - 8000 men and women were followed (the cohort). - They were followed for a long period of time (7 years). - They were free of the influence of the researchers during this period. - After this period, the weight gain was recorded (response variable).