ctools

I’ve been looking at the statistics of csresmap when using algorithm='SIGNIFICANCE’ (calculated here), and I ran into something odd when recreating the calculation in my python script.

The situation is: you’re calculating the significance of a bin that has 7 observed counts, and 6.9999999999999... modeled counts (7-<a really small number>).

$ python
>>> d = 7
>>> m = d - 1e-14
>>> print(sqrt(d*log(d/m) + m - d))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: math domain error
>>> d*log(d/m) + m - d
-8.881784197001252e-16

When calculating the significance (done here in csresmap), the value of

d*log(d/m) + m - d

becomes negative, and then the sqrt() throws an exception, ruining your wednesday night statistical experiments. I think its due to how python handles long floating point numbers, but I’m not sure.

Just to emphasize, I haven’t actually seen this happen yet in csresmap since I don’t have enough time to test it out. But I did see this when doing the same calculation with some MC statistical sims in a standalone python script, so I’m marking this as low priority, but it may be worth checking out.