Geometrical and statistical aspects of the accuracy of camera powder data

Reduction of random experimental errors and accounting for systematic errors require good experimental techniques and systematic handling of data. For the Straumanis–Ieviņš, van Arkel, and Bradley–Jay methods, in the absence of error, the sum of the readings of the measuring instrument for corresponding lines on either side of the exit (or entrance) hole would be constant. Drift of this sum with θ reveals certain types of systematic error resulting from geometrical aberrations having low symmetry; random scatter about the drift line gives an immediate measure of the random error in setting on a line. Unfortunately no such internal assessment is possible with the usual Guinier or diffractometer arrangements. The standard deviation of a measurement of line position is found to be show little variation with θ, though there is some indication that it is dependent on line intensity, being greater for very weak and very strong lines. The standard deviations of Bragg angles and lattice spacings or parameters are easily determined from that of the position measurements.