Stats: Independent sample t-tests

 

Last time we looked at how to do a one sample t-test analysis, today we look at the independent t-test. This examines whether the means of 2 separate samples are significantly different. An example might be a control group vs. an experimental group. The independent variable is condition (control, experimental), the dependent variable is the scores on a test. Our research question is – is the mean for the control group significantly different from the experimental group's mean? Our null hypothesis is that there will be no difference between the two groups. Let's do an example together. So open SPSS and enter the following data for your samples: 

Under Variable View (see tab at bottom of page), It should look like: 

Name	Type	Width	Decimals	Label	Values	Ignore the rest
condition	numeric	8	0	Condition	*(see below)	Ignore the rest
score	numeric	8	0	Score	None	Ignore the rest

 ·       For condition, Labels: 1= control; 2 = experimental

 Go back to Data View and enter the following: 

Condition	Score
1	5
1	6
1	8
1	2
1	4
2	11
2	15
2	9
2	7
2	12

 Go to Analyze/ Compare Means/ Independent samples t-test. Move your score variable into Test Variable. Move condition into Grouping Variable/ Define groups 1 and 2. Press ok.

 Your results should look like the following:

Group Statistics
	condition	N	Mean	Std. Deviation	Std. Error Mean
score	control	5	5.00	2.236	1.000
	experimental	5	10.80	3.033	1.356

 

Independent Samples Test
		Levene's Test for Equality of Variances		t-test for Equality of Means
		F	Sig.	t	df	Sig. (2-tailed)	Mean Difference	Std. Error Difference	95% Confidence Interval of the Difference
									Lower	Upper
score	Equal variances assumed	.433	.529	-3.442	8	.009	-5.800	1.685	-9.686	-1.914
	Equal variances not assumed			-3.442	7.356	.010	-5.800	1.685	-9.746	-1.854

 What does this mean? Your two samples are significantly different from each other (see highlighted areas). So let's write it up as you would in your paper: 

An independent sample t-test was conducted comparing the control group (M = 5; SD = 2.24) to the experimental group (M = 10.8; SD = 3.03). The result (t(8) =3.4, p= .009) indicates that there is a significant difference between the groups and the null hypothesis is rejected.

What happens if the results were NOT significantly different, as in this example:

 

Condition	Score
1	5
1	6
1	8
1	2
1	4
2	6
2	4
2	7
2	5
2	6

 You run this one on your own. Here is how I write it up:

An independent sample t-test was conducted comparing the control group (M = 5; SD = 2.24) to the experimental group (M = 5.6; SD = 1.14). The result (t(8) =-.54, p= .608) indicates that there is not a significant difference between the groups and the null hypothesis is retained. 

A great resource for SPSS is

Pallant, J. (2013). The SPSS Survival Manual, 5^th edition. Open University Press. 

Next time, we will look at paired sample t-tests. Do you have an issue or a question that you would like me to discuss in a future post? Send me an email with your ideas. leann.stadtlander@waldenu.edu