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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Take in mind is that what is meant is population variance, not sample variance. Those are different things, you might be using a wrong formula.
Sorry, London variance is correct. This test is working properly and confirmed by 5012 passing solutions (of which 353 in C++ which is - I suppose - your language).