The active twins were leaner and had higher HDL-cholesterol and apo A-I (a protein connected to HDL-particles) levels than their more sedentary brothers and sisters. The active twin had an average HDL-cholesterol level of 58 mg/dl, an apo A-I level of 154 mg/dl, and a body mass index (BMI) of 22.0 kg/m2. The corresponding values for the more sedentary twin were 53 mg/dl, 144 mg/dl, and 24.1 kg/m2, respectively. These differences between the active and sedentary twins were statistically significant, meaning that it is highly unlikely that they were due solely to chance. The figure to the right is a scatterplot of the twins' HDL-cholesterol levels. Each point represents one twin pair. A line drawn from the point down to the horizontal axis shows the HDL-cholesterol of the more sedentary twin. A line drawn to the vertical axis shows the HDL-cholesterol of the more active twin. If the twins always had exactly the same HDL-cholesterol, then all the points would fall along the diagonal. However, the points lie mostly above the line, confirming that the runners had higher levels of the good cholesterol than the sedentary twin. The plot shows something else as well. Despite their very different exercise levels, the twins remained very similar to each other. Their similarity is measured by a statistic called the correlation coefficient, often denoted by the letter "r". A value of "r" that is one would be a perfect correlation, and would be obtained if the HDL-cholesterol levels of the two twins were exactly the same. An r value of zero would be obtained if the HDL-cholesterol levels of the twins