At 12:37 AM 12/8/99 +0000, John Whittington wrote: ... >Indeed - but they also need to be sensible about how they 'manage' whatever >degree of precision they deem appropriate. I am, in particular, reminded >that rounding to an appropriate (and realistic) number of significant >figures generally should only be done *once*, at the end of all the >calculations - which themselves should utilise data to the full extent of >whatever precision is available. > >I have seen some very serious errors resulting from 'repeated stages of >rounding' ...

Again, Dr. John is right on the button. I would make one minor alteration.

The values input to the analysis should not exceed the precision of the measurements from which they are derived.

I had one case where a researcher had done a preliminary analysis which indicated a significant effect. Being unsure of the specification for some additional statistics, he asked me to take a crack at the data. I noticed that the data (in an Excel spreadsheet ;-) had about 7 decimal places, and asked about the precision of the measurements. I forget whether we decided that the (reaction time) measurements were really only accurate to 2 or 3 places. In any case, after rounding to one extra digit (3 or 4 places) the significant effect almost disappeared.

Now, this could easily have been detected by reviewing the results even if we kept all 7 digits. Nevertheless, keeping or displaying the original data with precision beyond the measurements can affect the way we think and talk about our research--even occasionally leading us to believe that insubtantial differences are significant.

-lary _______________________________________________________ Lary Jones % Statistical Computing Analyst Computing Services % .......................... Binghamton University % LJones@Binghamton.EDU Binghamton, NY 13902-6000 % (607) 777-2614