HP 54710A User's Reference Manual page 242

Measurements
Time-interval measurements
What if, as shown in figure 13-28, none of these samples coincide with the
10 percent or 90 percent level? If you use the time between the nearest
samples as an estimate of the rise time, the error could be as much as 125 ps,
which is 12.5 percent of the actual rise time, and the resolution could not be
better than 250 ps. You could get a better estimate of the rise time by simply
interpolating between the sample points with a straight line.
Figure 13–28
Linear Interpolation Between Samples Improves Accuracy of Rising Edge Measurement
13–57