# Confused when to use Population vs Sample standard deviation in engineering testing

When I run an test for something (say 10 trials) and want to find the standard deviation of all 10 trials, I am getting confused if I should use the sample or population standard deviation. My initial thought would be the sample standard deviation since I could run 10 more trials and now have more data points thus never having the complete population.

A lot of the examples I see online involve student grades or finance applications (which I never deal with) but I am having trouble finding a concrete answer on what to do when it is possible to run more tests and have more data points but using all the data points you have to get a standard deviation.

## 1 Answer

The two forms of standard deviation are relevant to two different types of variability. One is the variability of values within a set of numbers and one is an estimate of the variability of a population from which a sample of numbers has been drawn.

The population standard deviation is relevant where the numbers that you have in hand are the entire population, and the sample standard deviation is relevant where the numbers are a sample of a much larger population.

For any given set of numbers the sample standard deviation is larger than the population standard deviation because there is extra uncertainty involved: the uncertainty that results from sampling. See this for a bit more information: Intuitive explanation for dividing by $n-1$ when calculating standard deviation?

For an example, the population standard deviation of 1,2,3,4,5 is about 1.41 and the sample standard deviation is about 1.58.

• I guess I am still a bit confused. Say I wanted to the determine the hardness of a certain material and got 5 data points (1,2,3,4,5) would I use population or sample? Nothing is stopping me from running an additional 5 tests and getting more data points (6,7,8,9,10) – pnd1987 Aug 30 '20 at 22:34
• Are you intending to use your standard deviation as an estimate of the variability of the material hardness in general, or just the variability in your measurements? If it's the former then you have a sample and want to estimate standard deviation of the notional population that is the material in general. If it's the latter then you should use the population standard deviation. The fact that you say that there is nothing stopping you from obtaining more values means that you are sampling: use the sample standard deviation. – Michael Lew Aug 30 '20 at 23:18