Array formula error

Firstly, I would like to say this forum is super helpful and thank you everyone.

I am having trouble with a standard deviation array formula. Honestly, I don't really understand how to use it.

However, I got it right once but when I try to use it for other variables I keep getting #DIV/0!.

Please refer to tab 1.6 of the excel document below.

Thanks!

That would be because the columns you are checking don't have the word "successful" in them.

For example, this works:

Formula:

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