ISa

CORRECT is the scaling step of XDS.

An estimate for the overall quality of an experimental setup

A useful Unix command that prints a single number that depends on the overall quality of an experimental setup (beam, crystal, spindle, detector, cryo, software, ...) is

awk '/CORRECTION PARAMETERS/{for (i=0;i<9;++i) getline;print 1/($1*$2)^0.5}' CORRECT.LP

Why? Because this will give you the best (maximum) value of I/sigma(I) for any reflection in your dataset - even if your crystal is great, all reflections are bound to be worse than that.

Why does the command give you such a useful value? It just finds the line "CORRECTION PARAMETERS FOR THE STANDARD ERROR OF REFLECTION INTENSITIES", skips the next 9 lines, and grabs the values of "a" and "b" from CORRECT.LP. These values appear in the formula v(I)=a*(v0(I)+b*I^2) which is used by CORRECT to adjust the variances of the intensities, to match their experimental spread. For strong and well-measured reflections, the variance is dominated by the systematic error that is introduced by any beam or detector instability. For weak reflections, v0(I), the variance from counting statistics, dominates. The value for v(I) that the formula gives, will be higher than v(I)=a*b*I^2 . Therefore, I/sigma(I) = I/sqrt(v(I)) will be lower than 1/sqrt(a*b) which is what the Unix command prints out.

Sometimes however, e.g. if too few strong reflections exist in the dataset, b might come out negative. In that case the Unix command prints out "nan" which means "not a number" and indicates that it could not calculate the square root of a negative number.

If your crystal is badly split or broken, then of course a and b will be strongly influenced by the crystal quality. If your data reduction is not good (e.g. wrong spacegroup) then the values of a and b might not be reliable.

If your crystal is good (and no matter how good your crystal is!), then a and b will reflect the quality of the beamline. I have seen values lower than 20 for good crystals at bad beamlines. On the bright side, I have also seen a value of 87.6 for Z. Dauter's 0.98A Proteinase K (2ID8) sulfur-SAD data from J. Holton's APS/22-ID beamline.