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In step-scan diffraction measurements, the diffraction angle 2θ is an observation with standard uncertainty u(2θ). By the law of uncertainty propagation, u(2θ), typically 0.001 < u(2θ) < 0.004°, affects the standard uncertainty 
of the intensity y at each step 
, depending on the local slope 
, by ![u_{\rm total}^2(y_i) = u^2_{\rm Poisson}+[y'_iu(2\theta)]^2](//journals.iucr.org/a/issues/1998/02/00/sh0098//teximages/sh0098fi4.gif)
, where 
is the conventional Poisson statistics. For the intensity y at 2θ of steepest slope, is given by 
, where 
is the ratio of 
and 
, 
is the peak intensity and h the full width at half-maximum of the profile. The error of the intensities at individual steps modifies also the standard uncertainty of the integrated intensity: 
. As v depends on 
, it is evident that the importance of the correction increases with increasing count rates and decreasing line width. In most practical cases, contributes a multiple of Poisson statistics to the standard uncertainty of intensity. It will be shown that with a realistic weighting scheme the 
as well as the Durbin–Watson test become more meaningful.
of the intensity y at each step 
, depending on the local slope 
, by ![u_{\rm total}^2(y_i) = u^2_{\rm Poisson}+[y'_iu(2\theta)]^2](http://journals.iucr.org/a/issues/1998/02/00/sh0098//teximages/sh0098fi4.gif){kind=small}
, where 
is the conventional Poisson statistics. For the intensity y at 2θ of steepest slope, is given by 
, where 
is the ratio of 
and 
, 
is the peak intensity and h the full width at half-maximum of the profile. The error of the intensities at individual steps modifies also the standard uncertainty of the integrated intensity: 
. As v depends on 
, it is evident that the importance of the correction increases with increasing count rates and decreasing line width. In most practical cases, contributes a multiple of Poisson statistics to the standard uncertainty of intensity. It will be shown that with a realistic weighting scheme the 
as well as the Durbin–Watson test become more meaningful.
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