TEN CHECKS BEFORE YOU TRUST A NUMBER
From "Mastering XRR Fitting", Chapter 25. Ask these of every fitted number
(a thickness, a period, a sigma, a density) before you use it.

[ ]  1. Does the curve carry this number?
        A feature of the curve must move when the number changes.
        Look: Data > Assess XRR quality ...; the sensitivity map.   Ch 10, 13

[ ]  2. Were the data and the settings set for this curve?
        2theta or theta; Normalize (Auto); no smoothing; the range from
        theta_max; R min; Delta-theta from a scan of this curve.     Ch 14, 16

[ ]  3. Is the scale inside its window?
        Look: Chart Info bar, scale box. "at bound" = no: check the end
        of the range, then the model.                               Ch 14

[ ]  4. Does the fit pass its checks?
        Look: Result > Fit report ... Orders, edge, eight bands, chi2
        last. A fail is reported with its cause.                    Ch 18

[ ]  5. Is the model complete?
        No sigma far larger than its neighbors, no density on a limit
        standing in for a missing layer. Both modes fitted; the mode
        chosen on the drift.                                     Ch 17, 19

[ ]  6. Is the number clear of its limits?
        Look: Fitting Limits dialog, yellow cells. Widen and fit again
        until no run ends near a limit.                             Ch 15

[ ]  7. Do five runs agree on it?
        Spread over five runs smaller than the precision you claim,
        or report "undetermined". Report the best run, never a mean. Ch 16, 18

[ ]  8. Does the curve fix this number, or only a sum?
        Sums of layers that trade thickness; sigma as fitted; per-period
        values of a profile; a film's thickness only.        Ch 18, 21, 22

[ ]  9. Is it a density?
        Then it is a "free parameter, not a result".                Ch 20

[ ] 10. Is it written down with its settings?
        Result > Export fit results... right after the fit; the Fit
        report text; the project saved; the 22 report items.        Ch 23