EXFO FTB-7000 OTDR for FTB-200 v2 - Manual (Page 181)

Analyzing the Results Manually
OTDR 167
Getting Event Loss (Four-Point and Least-Square Approximation)
Getting Event Loss (Four-Point and
Least-Square Approximation)
Event loss (expressed in dB) is calculated by measuring the signal level
reduction in Rayleigh backscatter (RBS) caused by this event. Event loss
can result from both reflective and non-reflective events.
Two loss calculations are provided simultaneously: the four-point event
loss and the A-B LSA loss. Both calculations use the least-square
approximation (LSA) method to determine the event loss. However, the
four-point event loss is the preferred method and the one that corresponds
to the loss displayed in the events table.