Researchers examined middle-aged adults from 4 locations in the United States from 1987 to 1989. They measured blood pressure carefully, took blood samples for cholesterol levels, and asked questions about medical history and health behaviors. The researchers repeated these assessments at 3-year intervals 3 times (total of 4 assessments). At the third assessment, they took digital pictures of the retinas of the patients and measured the size of the vessels on the photographs. They then looked at people who developed high blood pressure between the third and fourth assessments to see how often people with and without narrowed retinal vessels developed high blood pressure. These analyses “controlled for” other factors, such as age, sex, smoking status, diabetes, cholesterol level, and blood pressure level, that might be associated with increased risk for developing high blood pressure.