In a study including cephalometric or dental model measurements, investigators often wish to examine the reliability of those measurements. Therefore, a percentage of the original measurements are repeated a few weeks later, and the 2 sets of measurements (original and repeated) are assessed for agreement. An often encountered but incorrect approach to assess agreement between continuous measurements is using a paired t test. To better explain why this method is incorrect, I will provide a simplified example. The Table shows the results of 2 scenarios (A and B) of 20 pairs of measurements conducted at 2 different time points and their differences. The data has been manipulated so that there is no difference between the means in either scenario A or B between the time 1 and 2 measurements (see mean values [= 10.45] at the end of the Table ). For reasons of simplification, variances of the means at the 2 time points are ignored.
|ID||Scenario A||Scenario B|
|Time 1||Time 2||Difference||Time 1||Time 2||Difference|