Hi Lu Yongzhi,
It has to be said it is not obvious. However, there are as far as I can
see at least three possible definitions of I/sigma, so here we go:
(1) I/sigma (calculated, corrected perhaps) for individual observations
- sigma based on counting statistics
(2) Mean(I) / Sigma(mean) - that is, including the improvement in the
measurement by including multiple observations
(3) Mean(I) / Spread - that is, the mean I value divided by the spread
(standard deviation?) of the observations - which is different to (1)
Quoting from pdb pages:
Item _reflns_shell.meanI_over_sigI_all
Description
The ratio of the mean of the intensities of all
reflections
in this shell to the mean of the standard uncertainties
of the
intensities of all reflections in this shell.
They do not include a definition for the "sd" version of this.
Reading through a handily found scala log file and comparing to the
harvesting stats, I find that:
meanI_over_sd_all = Mn(I)/sd - (2) above
meanI_over_sigmaI_all = I/sigma - (1) above
Hope this answers your question.
Cheers,
Graeme
________________________________
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Lu
Yongzhi
Sent: 19 January 2008 09:07
To: [log in to unmask]
Subject: [ccp4bb] difference between I over sigma_I and I over sd
Hi everyone,
When I scale the data using Scala, I don't know differences between
_diffrn_reflns.meanI_over_sigI_all 6.5475
_diffrn_reflns.meanI_over_sd_all 21.3013
in the statistics. I think that both "sigma" and "sd" have the same
meaning--"standard deviation". By the way, in the file header of PDB's,
what are the meanings of the numbers 5 and 1 in the row "HELIX", numbers
0, 1 and -1 in the row "SHEET". And in the row "SHEET", what's the
meaning of the last 6 columns.
HELIX 1 1 THR A 35 ASP A 37 5
3
HELIX 2 2 SER A 41 TYR A 46 1
6
HELIX 3 3 GLU A 68 ALA A 82 1
15 SHEET 1 C 6 GLU A 224 ARG A 232 0
SHEET 2 C 6 ILE A 214 GLU A 221 -1 N CYS A 217 O ARG A 229
SHEET 3 C 6 PHE A 183 VAL A 189 -1 N PHE A 187 O LEU A 218
SHEET 4 C 6 VAL A 302 LEU A 309 -1 O ILE A 306 N LYS A 184
SHEET 5 C 6 LYS A 254 HIS A 261 -1 N ILE A 258 O GLY A 305
SHEET 6 C 6 GLU A 280 PRO A 288 1 O ASP A 283 N VAL A 257
Thanks
Lu Yongzhi
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