Correlation in restricted ranges of data

In a study of 150 adult diabetic patients there was a strong correlation between abdominal circumference and body mass index (BMI) (r = 0.85).1 The authors went on to report that the correlation differed in different BMI categories as shown in the table.⇓ View this table: Correlation between abdominal circumference and body mass indeed (BMI) in 1450 adult patients with diabetes The authors’ interpretation of these data was that in patients with low or high BMI values (BMI 35 kg/m2) the correlation was strong, but in those with BMI values between 25 and 35 kg/m2 the correlation was weak or missing. They concluded that measuring abdominal circumference is of particular importance in subjects with the most frequent BMI category (25 to 35 kg/m2). When we restrict the range of one of the variables, a correlation coefficient will be reduced. For …