Just a wee addition to the above: if you really can't be arsed to read all the myriad postings on this thread, but want help in identifying your Sigma, at least track down Stirling Webb's post of 23 October 2004, which gives a potted history and a number of pointers about what to look for.

Alas, Stirling hasn't posted here for a very long time now, but we are all in his debt.