Use sample standard deviation.

[nishihatapalmer]

• We were using the population standard deviation, rather than the sample standard deviation.
• Since variability in a population is almost certainly a bit bigger than what was captured in a sample, we divide by one less than the number of samples, rather than the total number of samples.
• This biases the standard deviation to be slightly bigger which tends to give better estimates for the population as a whole.

[nishihatapalmer]

See: https://www.statology.org/population-vs-sample-standard-deviation/ for an explanation of the difference.