Sample and population standard deviation.
Standard deviation measures spread around the mean. Sample (s) divides by n−1; population (σ) divides by n.
Larger σ = more spread. About 68% of a normal distribution lies within 1σ of the mean.
The n−1 adjustment for sample standard deviation (Bessel's correction) exists because a sample tends to slightly underestimate the true population spread — dividing by a smaller number corrects for that bias.
5,7,3,7,10,2: mean 5.67, σ (pop) 2.69, s (sample) 2.94.
Sample (s) if your data is a sample of a larger population — usually the answer. Population (σ) only if you have the entire population.
Variance = σ². Std dev is variance's square root, which has the same units as the data — usually more interpretable.
Every value in the data set is identical — there's no spread at all around the mean.