Returns the onetailed probabilityvalue of a ztest. For a given hypothesized population mean, μ_{0}, ZTEST returns the probability that the sample mean would be greater than the average of observations in the data set (array) — that is, the observed sample mean.
To see how ZTEST can be used in a formula to compute a twotailed probability value, see "Remarks" below.
Syntax
ZTEST(array,μ_{0},sigma)
Array is the array or range of data against which to test μ_{0}
μ_{0} is the value to test.
Sigma is the population (known) standard deviation. If omitted, the sample standard deviation is used.
Remarks
 If array is empty, ZTEST returns the #N/A error value.
 ZTEST is calculated as follows when sigma is not omitted:
or when sigma is omitted:
where x is the sample mean AVERAGE(array); s is the sample standard deviation STDEV(array); and n is the number of observations in the sample COUNT(array).
 ZTEST represents the probability that the sample mean would be greater than the observed value AVERAGE(array), when the underlying population mean is μ_{0}. From the symmetry of the Normal distribution, if AVERAGE(array) < μ_{0}, ZTEST will return a value greater than 0.5.
 The following Excel formula can be used to calculate the twotailed probability that the sample mean would be further from μ_{0} (in either direction) than AVERAGE(array), when the underlying population mean is μ_{0}:
=2 * MIN(ZTEST(array,μ_{0},sigma), 1  ZTEST(array,μ_{0},sigma)).
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example
 Create a blank workbook or worksheet.
 Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 In the worksheet, select cell A1, and press CTRL+V.
 To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

A 
Data 
3 
6 
7 
8 
6 
5 
4 
2 
1 
9 
Formula 
Description (Result) 
=ZTEST(A2:A11,4) 
Onetailed probabilityvalue of a ztest for the data set above, at the hypothesized population mean of 4 (0.090574) 
=2 * MIN(ZTEST(A2:A11,4), 1  ZTEST(A2:A11,4)) 
Twotailed probabilityvalue of a ztest for the data set above, at the hypothesized population mean of 4 (0.181148) 
=ZTEST(A2:A11,6) 
Onetailed probabilityvalue of a ztest for the data set above, at the hypothesized population mean of 6 (0.863043) 
=2 * MIN(ZTEST(A2:A11,6), 1  ZTEST(A2:A11,6)) 
Twotailed probabilityvalue of a ztest for the data set above, at the hypothesized population mean of 6 (0.273913) 
