Hi @arthurits, thanks so much for this PR! This is fantastic, and I look forward to merging it in a few minutes!

Functions StDev are renamed to StdDev.

Note that I'll keep it StDev to be consistent with naming found in Python's standard library. I try to mimic the API of Python (and the numpy and matplotlib packages) where possible.

• https://docs.python.org/3/library/statistics.html#statistics.stdev

Thinking out loud about performance...

I'm tempted to make this an IReadOnlyList which will allow it to accept both List<double> and double[] and I think offer improved performance because .Count can be used instead of Linq's .Count() ... Probably the decision should be made with proper benchmarks to prove it's meaningfully faster, but I'm not really motivated to micro-optimize these methods at this time.

Similarly, I'll probably make the overloads private that accept the mean "for performance". Maybe it saves a few cycles by not calculating mean twice, but calculating mean isn't very costly considering that function is loaded with Sqrt() calls which are a lot more significant

Also I'm going to remove the parallel code here because parallel processing can be significantly slower for small datasets. If we benchmark it and I'm incorrect, we can add it back

One more thought, rather than passing a flag to switch between population or sample standard deviation, I'll probably make it two functions to mimic python

• https://docs.python.org/3/library/statistics.html#statistics.stdev
• https://docs.python.org/3/library/statistics.html#statistics.pstdev

[sorry about the message bomb here lol] ... thinking more out loud,

In researching Microsoft's naming of this stuff, it looks like they favor StandardDeviation() and StandardDeviationP() which is a few more keystrokes but may be more readable be a better reference than Python APIs

• https://learn.microsoft.com/en-us/dotnet/api/system.data.entity.core.objects.entityfunctions.standarddeviation?view=entity-framework-6.2.0
• https://learn.microsoft.com/en-us/dotnet/api/system.data.entity.core.objects.entityfunctions.standarddeviationp?view=entity-framework-6.2.0

I look forward to merging it in a few minutes!

A few minutes turned into a few hours - what a rabbit hole! I think this ended in a fantastic place though and I'm merging now. Thanks again for your help getting this started @arthurits!

There's room for benchmark-driven micro-optimization in the future, but I'm happy where this is.

Hi @swharden. Although it looked quite straightforward, it turns out there were many points to address.
The final code looks great!