Question-and-Answer Resource for the Building Energy Modeling Community
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In this case, when you are estimating the variance you don't know what the real mean is so you have to use an estimate of the mean instead of the true mean. Estimating the mean reduces the degrees of freedom (number of samples) by one so you use N-1 instead of N which results in a slightly larger estimate of the variance. This link might help a bit:
https://stats.libretexts.org/Bookshelves/Introductory_Statistics/Book%3A_Introductory_Statistics_(Lane)/10%3A_Estimation/10.02%3A_Degrees_of_Freedom
