There are no random tests.

ϕ = 
  n11 * n00 - n10 * n01
  ----------------
  sqrt√(n1 * n0 * n1 * n0)

This formula is wrong; the denominator should be sqrt((n00 + n01) * (n10 + n11) * (n00 + n10) * (n01 + n11)).

This problem has been copied from Eloquent Javascript which is licensed under Creative Commons attribution-noncommercial license.

In which it is clearly mentioned:

You are free to:

    Share — copy and redistribute the material in any medium or format
    Adapt — remix, transform, and build upon the material

    The licensor cannot revoke these freedoms as long as you follow the license terms.


Under the following terms:

    Attribution — You must give appropriate credit, provide a link to the license, and 
    indicate if changes were made. You may do so in any reasonable manner, but not in any
    way that suggests the licensor endorses you or your use. 

So you've to give the credit in the description.

This is copied from Eloquent Javascript - maybe you should at least credit it?

Because those are different things 🙂

a and a, b and B. Therefore, there are 2 characters that occur in duplicate.
I hope that helps.

The bottom-most example test cases looked something like this:

Test.assertEquals(correlations2.beer == 0.25);

The double equal signs should be replaced with a comma.

But since the kata is in cooperation mode, I edited it myself. :-)

Thanks for a nice kata.

What are you saying the issue is?

Thanks for pointing that one out Steve! Feel free to implement the table if you think it is more clear. This Kata is in collaboration mode.

Apart from Steve's issues, the last provided test cases are using Test.assertEquals wrongly like Test.expect. They should be:

Test.assertEquals(correlations2.beer, 0.25);
Test.assertEquals(correlations2.lasagna, -0.5);
Test.assertEquals(correlations2.running, 0.25);

This is an interesting and educational kata. Thanks for making it!

There are a few minor issues with the description:

The notation explanation could be clearer. You could borrow the table from Wikipedia, which is pretty clear:

The markdown for the table above looks like this:

|       | y = 1 | y = 0 | total |
|-------|-------|-------|-------|  
| x = 1 |  n11  |  n10  |  n1●  |
| x = 0 |  n01  |  n00  |  n0●  |
| total |  n●1  |  n●0  |  n    |

Also, the second assertion in the sample tests expects the correlation to be -.05, but it should be -0.5.