[Python-Dev] Stop using timeit, use perf.timeit! (original) (raw)

Steven D'Aprano steve at pearwood.info
Mon Jun 13 00:29:31 EDT 2016

On Sat, Jun 11, 2016 at 07:43:18PM -0400, Random832 wrote:

On Fri, Jun 10, 2016, at 21:45, Steven D'Aprano wrote: > If you express your performances as speeds (as "calculations per > second") then the harmonic mean is the right way to average them.

That's true in so far as you get the same result as if you were to take the arithmetic mean of the times and then converted from that to calculations per second. Is there any other particular basis for considering it "right"?

I think this is getting off-topic, so extended discussion should probably go off-list. But the brief answer is that it gives a physically meaningful result if you replace each of the data points with the mean. Which specific mean you use depends on how you are using the data points.

http://mathforum.org/library/drmath/view/69480.html

Consider the question:

Dave can paint a room in 5 hours, and Sue can paint the same room in 3 hours. How long will it take them, working together, to paint the room?

The right answer can be found the long way:

Dave paints 1/5 of a room per hour, and Sue paints 1/3 of a room per hour, so together they paint (1/5+1/3) = 8/15 of a room per hour. So to paint one full room, it takes 15/8 = 1.875 hours.

(Sanity check: after 1.875 hours, Sue has painted 1.875/3 of the room, or 62.5%. In that same time, Dave has painted 1.875/5 of the room, or 37.5%. Add the percentages together, and you have 100% of the room.)

Using the harmonic mean, the problem is simple:

data = 5, 3 # time taken per person mean = 3.75 # time taken per person on average

Since they are painting the room in parallel, each person need only paint half the room on average, giving total time of:

3.75/2 = 1.875 hours

If we were to use the arithmetic mean (5+3)/2 = 4 hours, we'd get the wrong answer.

-- Steve

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