EXFO MAX-700 - Manual (Page 184)

Analyzing the Results Manually
172 MAX-700
Getting Event Loss (Four-Point and Least-Square Approximation)

Four-point event loss: the LSA method is used to fit a straight line to the
backscatter data within the two regions defined by markers a, A and
b, B, that is over the regions to the left and to the right of the event
bordered by markers A and B, respectively.
The two fitted lines are then extrapolated toward the center of the
event and the loss event is directly read from the drop in power
between the two lines.
Four-point
event loss
a
b