issue_comments: 850690985

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https://github.com/pydata/xarray/pull/5390#issuecomment-850690985 https://api.github.com/repos/pydata/xarray/issues/5390 850690985 MDEyOklzc3VlQ29tbWVudDg1MDY5MDk4NQ== 56925856 2021-05-28T21:43:52Z 2021-05-28T21:44:12Z CONTRIBUTOR

is it just

cov = _mean(da_a * da_b) - da_a.mean(dim=dim) * da_b.mean(dim=dim)

I think you'd still have to normalize the second term by 1 / (valid_count). However, I just tried both of these approaches and neither pass the test suite, so we may need to do more thinking... 

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