Andrew,

I totally agree that it's not going to be that clear to folks -- but I'm as stumped as you as to how to make it clear without getting really wordy.

Also, I think the percent error use case is infrequent, more likely would be that a relative tolerance of 1e-8 means that the numbers are the same to within about 8 significant decimal figures. After all, not many people think in terms of 0.0000001%

Suggestions gladly accepted!

-Chris

On Thu, Jan 22, 2015 at 7:30 PM, Andrew Barnert <abarnert@yahoo.com> wrote:

Overall I like it, but I'm not sure the help on the tol parameter is clear enough for people who don't already know what they want--in other words, the very people this function should be helping.

In my experience, novices understand relative tolerance immediately if you put it in terms of "within X% of expected", but don't always understand it if you put it in terms of "within X * expected" or, worse, "relative to the magnitude of the expected value". Just using % in there somewhere makes people get the concept.

Unfortunately, since the API doesn't actually use a percentage--and shouldn't--I'm not sure how to get this across in a one-liner in the help. You can always add something like "(e.g., a relative tolerance of .005 means that the actual value must be within 0.5% of the expected value)", but that's way too verbose.

(Also, I should note that the people I've explained this to have mostly been people with a US 1960-1990-style basic math education; I can't be sure that people who learned in another country, or in the post-post-new-math era in the US, etc. will respond the same way, although I do have a bit of anecdotal evidence from helping a few people on forums like StackOverflow that seems to imply they do.)

Sent from a random iPhone

On Jan 22, 2015, at 16:40, Chris Barker <chris.barker@noaa.gov> wrote:

> is the relative tolerance -- it is the amount of error
> allowed, relative to the magnitude of the expected value.

Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
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Chris.Barker@noaa.gov