HP 54710A User's Reference Manual page 193

Measurements
Then It Calculates Top and Base
The greatest number of data occurrences in the top half of the histogram
corresponds to the top. If the occurrence count is less than a predetermined
statistically significant value, the top defaults to the value of the absolute
maximum. Likewise, the base represents the level with the greatest number
of occurrences in the bottom portion of the histogram. If the occurrence
count is less than a statistically significant value, the base defaults to the
value of the absolute minimum. This may be the case for very irregular
waveforms or those in which the voltage rises or falls slowly over time.
This selection technique prevents the top and base from jumping around on
waves with even histogram distributions such as triangle waves. It also allows
the detection of the top and base for low duty cycle signals. The oscilloscope
also allows you to define the top and base directly in the define measure
menu.
Once the top and base measurements are complete, Vtop, Vbase, and Vamp
measurements can be made.
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