Tables for
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2006). Vol. B. ch. 2.1, p. 198   | 1 | 2 |

Section Intensities scaled to the local average

U. Shmuelia* and A. J. C. Wilsonb

aSchool of Chemistry, Tel Aviv University, Tel Aviv 69 978, Israel, and bSt John's College, Cambridge, England
Correspondence e-mail: Intensities scaled to the local average

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When the [G_{i}]'s are a subset of the [H_{i}]'s, the beta distributions of the second kind are replaced by beta distributions of the first kind, with means and variances readily found from Table[link]. The distribution of such a ratio is chiefly of interest when Y relates to a single reflection and Z relates to a group of m intensities including Y. This corresponds to normalizing intensities to the local average. Its distribution is [p(I/\langle I \rangle)\;{\rm d}(I/\langle I \rangle) = \beta_{1}(I/n\langle I \rangle\hbox{; } 1,n-1)\;{\rm d}(I/n\langle I \rangle) \eqno(] in the acentric case, with an expected value of [I/\langle I \rangle] of unity; there is no bias, as is obvious a priori. The variance of [I/\langle I \rangle] is [\sigma^{2} = {{n-1}\over{n+1}}, \eqno(] which is less than the variance of the intensities normalized to an `infinite' population by a fraction of the order of [2/n]. Unlike the variance of the scaling factor, the variance of the normalized intensity approaches unity as n becomes large. For intensities having a centric distribution, the distribution normalized to the local average is given by [{p(I/\langle I \rangle)\;{\rm d}(I/\langle I \rangle) = \beta_{1}[I/n\langle I \rangle\hbox{; } 1/2,(n-1)/2]\;{\rm d}(I/n\langle I \rangle),} \eqno(] with an expected value of [I/\langle I \rangle] of unity and with variance [\sigma^{2} = {{2(n-1)}\over{n+2}}, \eqno(] less than that for an `infinite' population by a fraction of about [3/n].

Similar considerations apply to intensities normalized to [\Sigma] in the usual way, since they are equal to those normalized to [\langle I \rangle] multiplied by [\langle I \rangle/\Sigma].

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