You should see that the lattice parameter refines within the precision of the results to approximately 10.602 Å in each case, but that the 2θ_zero error varies in each case. One of the data sets corresponds to a well-aligned instrument, while the other two patently do not. It is important to realize that all three refinements produce an accurate result within the precision determined by the esd's despite the fact that the 2θ_zero error correction is being misused; the latter is actually compensating for errors due to sample height displacment effects. Now repeat the exercise, but edit the form so that the 2θ_zero error is not refined. This time you will observe large variations in the refined value of the cubic cell parameter a: in two cases the value obtained will be very inaccurate, though this is not evident from the cell parameter esd'd obtained from the refinement.

For the two data sets where the sample height is incorrect, the accuracy of the value of a is much improved by using the high-angle data only for the refinement. You should be able to see in the list of the differences between observed and calculated angles that the error due the sample height displacement effect is still present, but that its effect is much reduced. You may like to try the same refinements again, but with 2θ_zero allowed to refine.

Although accuracy of the unit-cell parameters is clearly effected by instrumental effects such as sample displacement and instrument misalignment, it can also be affected by other factors such as sample purity, transparency, or even absorption: different samples may have slightly different lattice parameters; hence the need for standard samples as described earlier. Refinement of the lattice parameters of organic compounds is likely to result in inaccurate values when deep flat-plate sample holders are used; also small peak shifts may occur in transmission geometry for samples which absorb heavily, so beware of potential inaccuracies under these conditions too.