PPT Slide
Note from Bob von Dreele on shft
“Choice of "shft" vs "ZERO" is most important.
Diffractometers are aligned to make ZERO=0.000 and the sample is "never" in the right place to make shft=0.0. So always refine shft for Bragg-Brentano data. Synchrotron data is different as the zero can be off and other systematic effects will look like "shft" or "trns". If sample is transparent on B-B diffractometers (especially pharmaceuticals!) then trns should be refined.”