Comparing independently determined structures

Ordered weighted differences between matching parameters in independently determined structures, the experimental deviates, follow a Gaussian distribution only if both determinations are effectively subject solely to the influence of random effects and all uncertainties in the parameters are reliably evaluated and propagated. Departures from Gaussian are readily detectable by plotting experimental deviates against corresponding normal probability deviates; a normal distribution produces a linear array of unit slope and zero intercept. Application of this procedure to the results of an X-ray and a neutron diffraction determination of the structure of Ni(ND3)(4)(NO2)(2) at similar to 11 K [Iversen, Larsen, Figgis, Reynolds & Schultz (1996). Acta Cryst. B52, 923-931] reveals uncertainties in both experiments to be underestimated by a factor of 2 for the atomic coordinates and 1.5 for the atomic displacement parameters (ADPs). In addition, two ADP deviates depart significantly from the remaining twenty that are normally distributed; the difference between X-ray and neutron values for one N-atom ADP is highly significant. Uncorrected parameter problems introduced into further modeling may lead to false inferences.