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Paired t-test Using Microsoft Excel

 
These tutorials briefly explain the use and interpretation of standard statistical analysis techniques. The examples include how-to instructions for Excel. Although there are different version of Excel in use, these should work about the same for most recent versions. They also assume that you have installed the Excel Analysis Pak which is free and comes with Excel (Go to Tools, Addins... if it is not already installed in your version of Excel.)

See www.stattutorials.com/EXCELDATA for files mentioned in this tutorial, © TexaSoft, 2008

Performing a Paired t-test in Excel

To compare two paired values (such as in a before-after situation) where both observations are taken from the same or matched subjects, you can perform a paired t- For example, suppose your data contained the variables BEFORE and AFTER, (before and after weight on a diet), for 8 subjects.  The hypotheses for this test are:

Ho: mLoss = 0 (The average weight loss was 0)

Ha: mLoss ≠ 0 (The weight loss was different than 0)

 

For example, the following weight loss data is used in this example

(DIET.XLS)

 

Before

After

162.00

168.00

170.00

136.00

184.00

147.00

164.00

159.00

172.00

143.00

176.00

161.00

159.00

143.00

170.00

145.00

1.     To perform a paired t-test, select Tools/ Data Analysis / t-test: Paired two sample for means.

2.     In the t-test: Paired two sample for means dialog box: For the Input Range for Variable 1, highlight the 8 values of Score in group “Before” (values from 162 to 170). For the input range for Variable 2, highlight the eight values of Score in group “After” (values from 168 to 145). For now, leave the other items at their default selections. This dialog box is shown below. Click OK. This dialog box is shown below:

 

  

3.     The results are shown in the output below:

t-Test: Paired Two Sample for Means

 

 

 

 

 

 

Variable 1

Variable 2

Mean

169.625

150.25

Variance

65.125

121.9285714

Observations

8

8

Pearson Correlation

-0.176747772

 

Hypothesized Mean Difference

0

 

df

7

 

t Stat

3.706873373

 

P(T<=t) one-tail

0.003792994

 

t Critical one-tail

1.894578604

 

P(T<=t) two-tail

0.007585988

 

t Critical two-tail

2.364624251

 

 

Thus, the two-tail p-value for this t-test is p=0.008 (.007585988) and t=3.71.

Excel actually does a poor job providing what you need to report the results of this test – for a more complete understanding, you need to realize that the paired t-test is actually a test on the DIFFERENCE between the two values. Thus, to make this a better analysis, first calculate the difference between BEFORE and AFTER, creating the following new column called “DIFF” using a formula such as =A2-B2 in cell C2 and copying the formula for the appropriate remaining cells in the worksheet. Notice also that the average difference is calculated (19.38) 

Before

After

DIFF

162.00

168.00

-6.00

170.00

136.00

34.00

184.00

147.00

37.00

164.00

159.00

5.00

172.00

143.00

29.00

176.00

161.00

15.00

159.00

143.00

16.00

170.00

145.00

25.00

Average Diff=

19.38

Look back up at the original hypotheses – what you are testing is that the average loss is different than zero (0). Thus, the t-test is actually testing to determine if the value 19.38 is sufficiently different from 0 to claim significance. Thus, the number you are interested in most is the average difference (loss) and not as much as the individual means of Before and After.

Therefore to report these results properly, you need the mean difference and standard deviation. You can get this be calculating descriptive statistics on the difference values. (Tools/Data Analysis/ Descriptive Statistics) – choose the Summary Statistics and 95% confidence interval options. The results in the following output: 

Column1

 

 

Mean

19.375

Standard Error

5.226776868

Median

20.5

Mode

#N/A

Standard Deviation

14.78355747

Sample Variance

218.5535714

Kurtosis

-0.52419581

Skewness

-0.575291944

Range

43

Minimum

-6

Maximum

37

Sum

155

Count

8

Confidence Level(95.0%)

12.35936334

Notice that the mean divided by the standard error (19.375/5.227 = 3.71) is same as the value of the “t Stat” in the previous table. Another piece of information that is usually reported is a 95% confidence interval. Using the Confidence Level (95%) value of 12.359 in the table, the confidence interval is the mean plus or minus this value. Thus, a 95% C.I. about Mean Difference is (7.01, 31.74).

To report these results in a journal article, you could use something like this:

“A paired t-test was performed to determine if the diet was effective.

The mean weight loss (M=19.38, SD =14.784, N= 8) was significantly greater than zero,  t(7)=3.71, two-tail p = 0.008, providing evidence that the diet is effective in producing weight loss. A 95% C.I. about mean weight loss is (7.01, 31.74).”

NOTE: The researcher should interpret the results using his or her knowledge of the subject matter – thus giving the variability of the sample, the weight loss could have been as low as an average of 7 pound to a high of 32 pounds (see the 95% confidence interval). If it is as low as 7 pounds, would this still mean that the diet was effective (in terms of the researcher’s experience.)?

NOTE: The test could have also been performed as a one-tail test. If so, use the appropriate t-statistic and p-value from the Excel table. 

NOTE: Also, you could do this test using an hypothesized value of the difference other than zero – although zero is almost always used. Excel provides the opportunity to enter another hypothesized value to test in the paired t-test dialog box.

 

End of tutorial

See http://www.stattutorials.com/EXCEL

 


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