Thanks Raymond.

Apologies for commenting here instead of at the PR.

While I've been fighting with more intermittedly broken than usual 
internet access, Github has stopped supporting my browser. I can't 
upgrade the browser without upgrading the OS, and I can't upgrade the OS 
without new hardware, and that will take money I don't have at the moment.

So the bottom line is that while I can read *part* of the diffs on 
Github, that's about all I can do. I can't comment there, I can't fork, 
I can't make push requests, half the pages don't load for me and the 
other half don't work properly when they do load. I can't even do a git 
clone.

So right now, the only thing I can do is comment on your extensive 
documentation in statistics.rst. That's very nicely done.

The only thing that strikes me as problematic is the default value for 
sigma, namely 0.0. The PDF for normal curve divides by sigma, so if 
that's zero, things are undefined. So I think that sigma ought to be 
strictly positive.

I also think it would be nice to default to the standard normal curve, 
with mu=0.0 and sigma=1.0. That will make it easy to work with Z scores.

Thanks again for this class, and my apologies for my inability to 
follow the preferred workflow.