Z-Test Two Sample for Means in Excel
QI Macros Add-in for Excel Makes Conducting Z tests a Snap
When to use the Z-Test two sample for means
- To compare population and sample means to determine if there is a significant difference.
- To compare means between two samples.
- To compare the mean of one sample to a given constant.
The Z-test is typically used in evaluating the results of standardized tests. Are the results from a sample of students outside of or within the standard test performance?
This example was taken from Statistical Analysis in Excel for Dummies by Joseph Schmuller.
Imagine a new training technique designed to increase IQ. Take a sample of 25 people and train them using the new technique. Take another sample of 25 people and give them no special training. Input the results in Excel.
To conduct a z test using QI Macros add-in:
- Select the data, then click on QI Macros Menu-> Statistical Tools -> f and t tests, then Z-Test two sample for means test:
- QI Macros will prompt you to enter:
- a significance level (default = 0.05):
- a hypothesized mean difference (in this case 0 ):
- QI Macros will calculate the variance for each data set selected and give you the opportunity to type in another value if appropriate.
- Q Macros will perform the z test calculations AND interpret the results for you:
Interpreting the z test Results
QI Macros compares the p-value (0.192) to the significance level (0.05) and interprets the result for you. In this example:"Cannot Reject the Null Hypothesis (Accept the Null Hypothesis) because p > 0.05 (Means are the Same/Means are not Different)."
- The null hypothesis H0 is that the mean difference = 0
or in other words the means are the same
- The alternative hypothesis Ha is that the mean difference is > 0
or in other words that the mean of the trained population is larger
|test statistic > critical value
(i.e. z> zcrit)
|Reject the null hypothesis|
|test statistic < critical value
(i.e. z< zcrit)
|Cannot Reject the null hypothesis (Accept the null hypothesis)|
|p value < a||Reject the null hypothesis|
|p value > a||Cannot Reject the null hypothesis (Accept the null hypothesis)|
Since the null hypothesis is that the means are the same this is a two-sided test. Therefore, use the two-tail values for your analysis.
Since the z statistic < zcritical (1.305 < 1.960) and p value > a ( 0.192> 0.05) , we cannot reject the null hypothesis (accept the null hypothesis) that the means are the same.
To do a one-sample z-test on the New data above, use native Excel formulas in an empty cell:
- Gives p=0.015
- Since the p-value is < 0.05, we can Reject the Null Hypothesis (Means are Different/Means are not the Same).
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