Helen Genevier (asker) Apr 3, 2009:

Thanks Zareh That's really helpful, thanks. I had never heard this term before and obviously without understanding that bit you don't understand the sentence :-)

Zareh Darakjian Ph.D. Apr 2, 2009:

analytical / analytic expression Hi Helen:

As you know, this is an expression in closed form. Now this expression contains a number of parameters. (Those are the constants mentioned). Those can be determined by doing a least-squares or partial-least squares fit to experimental data. When the fit is quite good, then one can extract the values of those constants. One example is the determination of temperature factors in X-ray diffraction experiments (subsequent to the experiment). Probably you are an expert in these matters :).