What is a good standard deviation?

'Good' depends entirely on context. Low SD means data points are close to the mean (precise). High SD means data is spread out (variable). For test scores, SD of 10–15 on a 100-point scale is typical.

What is the empirical rule (68-95-99.7)?

For normally distributed data: 68% of values fall within 1 SD of the mean, 95% within 2 SDs, and 99.7% within 3 SDs. This is also called the '3-sigma rule.'

When should I use population vs sample standard deviation?

Use population SD when you have the entire dataset (e.g., all 30 students in a class). Use sample SD when your data is a subset of a larger population (e.g., surveying 500 of millions of people).

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