Sample or population?
This is where most marks are lost. The sample standard deviation Sx divides by n − 1; the population standard deviation σx divides by n. The smaller divisor makes Sx the larger of the two, and the gap matters most when you have few values.
| Your data is… | Use | Divides by |
|---|---|---|
| A sample from a bigger group | Sx | n − 1 |
| Every member of the group | σx | n |
| Unstated in the question | Sx is the safer default | n − 1 |
Doing it on the TI-84
Type your values into L1 with STAT → 1: Edit, quit, then STAT → CALC → 1: 1-Var Stats. The full walkthrough is in the standard deviation guide, and every menu item is listed in the manual.
Frequently asked questions
Should I use Sx or σx?
Use Sx when your numbers are a sample taken from a larger group, which is almost always the case in coursework. Use σx only when the numbers are the entire population, such as the test scores of every student in one class when that class is the whole thing you care about.
Why does the TI-84 show two standard deviations?
1-Var Stats prints both because it cannot know which one you need. Sx divides by n − 1 and σx divides by n, so Sx is always the slightly larger number. Look at the wording of the question to decide.
Why is my TI-84 showing ERR: DIM MISMATCH?
That usually means a stat plot is switched on and pointing at lists of different lengths, or L1 is empty. Press 2nd then Y= and choose 4: PlotsOff, then run 1-Var Stats again.
How do I get these statistics on the calculator itself?
Press STAT, choose 1: Edit and type your values into L1. Press 2nd then MODE to quit, then STAT, arrow to CALC and choose 1: 1-Var Stats. The simulator on this site does the same thing, or you can type 1-Var Stats L1 straight onto the home screen.