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Independent Samples t-Test

Introduction

An independent-samples t-test compares the means between two unrelated groups, such as comparing the difference between class 1 and class 2, another very common example would be used to compare a binary gender difference.

If you have more than three groups consider using a One-Way Between-Subjects ANOVA

It is considered a parametric test and is only suitable for parametric data. To check if your data is parametric, please read our dedicated guide: Parametric or Not Guide (PDF)

If your data is non-parametric you should consider using a Mann-Whitney U-Test.

 

Test Procedure

  1. Click Analyze > Compare Means > Independent-Samples T Test

    Step 1: Click Analyze > Compare Means > Independent-Samples T Test

  2. Within the Independent-Samples T Test box, select the test variable or dependent variable you are analysing and move it to the test variable box. Then move your independent/grouping variable into the “Grouping Variable” box.

    Step 2: Within the Independent-Samples T Test box, select the test variable or dependent variable you are analysing and move it to the test variable box. Then move your independent/grouping variable into the “Grouping Variable” box.

  3. Select Define Groups and specify the groups you are going to test. In this example I am testing Class 1 against Class 2 therefore my groups are numbered 1 and 2.

    Step 3: Select Define Groups and specify the groups you are going test. In this example I am testing Class 1 against Class 2 therefore my groups are numbered 1 and 2.

  4. Select Continue or OK

 

Results

 Independent Samples t-Test Results Output Example

SPSS will generate three tables, for this test we need two, the Group Statistics and the Independent Samples Test:

Group Statistics

This table shows a selection of descriptive statistics: the sample size of each group (N), the mean of each group (Mean), and the standard deviation of each group (Std. Deviation), best practice is to report them all.

Independent Samples Test

This table shows the specific test results including the t-statistic (t), the degrees of freedom (df) the two-tailed significance or p-value (Two-Sided p), and the 95% Confidence Interval (95% Confidence Interval of the Difference).

 

Reporting the Results in APA Formatting

English scores of class 1 and class 2 students were compared. On average, class 1 students (Mean = 6.00, SD = 1.77) scored higher than class 2 students (Mean = 5.00, SD = 2.62). An independent-samples t-test indicated this difference, 1.00, 95%CI [-1.40, 3.40] was not statistically significant, t(19) = 0.89, p=.386