The way I'd encode this for a database is to have the program ask for two magnitudes, one for the lower bound and one for the upper bound.

If either one is left blank or made zero (blank would convert to zero internally) it would be assumed as open ended.

For example:
3.5 6.8 - This is a specific range with lower and upper bounds
0.0 6.8 - This means any quake magnitude up to and including 6.8
3.5 0.0 - This means any quake magnitude 3.5 and above
0.0 0.0 - This should not be accepted and request correction from the user

HOWEVER!!! (there's always a however) What about negative magnitude quakes? I mean, there is such a thing as a mag zero or even negative magnitude quake. I've seen a few in the catalogs.

But, I think it's safe to discard this tiny technicality and presume 0.0 is the bottom end.

Other thoughts: x.x is sufficient accuracy; if input is single digit assume x.0; all these rules should be fully documented, perhaps with pop up help boxes?

Magnitude scale? I agree that a magnitude scale should NOT be given.

But which scale is used for testing? Although this is not an issue for the database that we're helping Michael with, you may want to consider the following idea, Roger, for your evaluations.

Obviously if there is only one mag available, that's the one to use. But what if there's more than one? I think you've said you use the highest of any available? That's one idea. But are not some magnitudes "more correct" than others? Perhaps a ranking of correctness can be determined and in the case of multiple magnitudes, the most correct is chosen?

Brian