Jacco van de Streek a écrit :
Example 1. Background subtraction.
Measured: Iobs = 100 counts, so ESD = SQRT(100) = 10.
The background at that point is determined to be, say,
102. The nett intensity at that point is then 100 -
102 = -2. However, the ESD of the point does not
change as a result of the background subtraction, and
is still 10. Plotting -2 / 10 gives the correct plot.
Jacco,
here you suppose that the uncertainty (s.u.) ESDbgr of the background is
zero. That's usually not true. If your background Ibgr is determined
with the s.u. ESDbgr and your total intensity Iobs with the s.u.
ESDIobs, then the s.u. ESDnet of the net intensity Inet is not
SQRT(Iobs) but
ESDnet = SQRT(ESDIobs**2 + ESDbgr**2) = SQRT(Iobs + Ibgr)
if we suppose the Poisson statistics for the background too.
In your example ESDnet is not 10 but SQRT (100 + 102)~ 14
best regards
Radovan
--
Radovan Cerny
Laboratoire de Cristallographie
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