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?

z Test Example: New Training Technique Designed to Increase IQ

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:

  1. Select the data, then click on QI Macros Menu-> Statistical Tools -> and select the Z-Test two sample for means test:

  2. QI Macros will prompt you to enter:
    • a significance level (default = 0.05):
    • a hypothesized mean difference (in this case 0 ):
  3. QI Macros will calculate the variance for each data set selected and give you the opportunity to type in another value if appropriate.
  4. 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 because p > 0.05 (Means are the same.)"

  • 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
p value < a Reject the null hypothesis
p value > a Cannot Reject 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 that the means are the same.

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One-Sample Z-Test

To do a one-sample z-test on the New data above, use native Excel formulas in an empty cell:

  • =ztest(A1:A26,100)
  • Gives p=0.015
  • Since the p-value is < 0.05, we can reject the null hypothesis.

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