A study was interested in determining if walking 2 miles a day lowered someone's blood pressure. Twenty people's blood pressure was measured. Then, ten of these individuals were randomly selected from the initial 20 people. These ten were told to walk 2 miles a day for 6 weeks and to eat as they normally did. The other ten were told to eat as they normally would. After six weeks, their blood pressure levels were measured again. What type of study is this?.