The Excel Z.TEST function returns the one-tailed probability value of a z-test — the chance of observing a sample mean greater than the value x if the true population mean were x.
Syntax
| Argument | Description | |
|---|---|---|
array | Required | The array or range of data to test against the hypothesized mean. |
x | Required | The value to test — the hypothesized population mean. |
sigma | Optional | The known population standard deviation. If omitted, Excel uses the sample standard deviation of array. |
How to use it
Z.TEST answers a one-tailed question: given your data, how likely is it that the population mean is at least x? It returns P(Z > z), where the z-score is built from the sample mean, x, and the standard deviation.
If you omit sigma, Excel substitutes the sample standard deviation of the array. Supply sigma only when the population standard deviation is genuinely known. For a two-tailed probability, double the smaller tail: =2*MIN(Z.TEST(array,x), 1-Z.TEST(array,x)).
Reading the result: a small value (say below 0.05) means the sample mean sits well above x — evidence that the true mean exceeds x. A value near 0.5 means the sample mean is right around x.
Try it: interactive demo
Pick a Z.TEST example to see the formula and its result.
Practice workbook
Frequently asked questions
Is Z.TEST one-tailed or two-tailed?
x. For a two-tailed test use =2*MIN(Z.TEST(array,x), 1-Z.TEST(array,x)).What happens if I leave out sigma?
sigma only when the population standard deviation is actually known.What is the difference between Z.TEST and ZTEST?
Z.TEST is the modern name (Excel 2010+); ZTEST is the legacy compatibility version. Use Z.TEST in current workbooks.When should I use Z.TEST instead of T.TEST?
T.TEST) is more appropriate.Master functions like this in one day
This page covers one function. Our Excel Formulas and Functions class covers the 30 that matter most — live, hands-on, taught by professionals in Dallas–Fort Worth, Houston, Austin, Oklahoma City, Denver, or online.
See the Formulas & Functions Class