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