Decide if your test is one-tailored or two-tailored.

Before you embark on your SPSS test for independent samples, there are three important things that you should do: SPSS is one of the conventional technologies used to run the t-test. The company can do a t-test to confirm is the results are repeatable for an entire population. Although the results may show that the drug is working, it could be due to a fluke. Then there is the group taking the drug which might show a life expectancy of 6+ years.

This group is normally given a placebo or sugar pill and might show an average life expectancy of let say 5+ years. In this experiment, they will have a control group. For example, a company manufacturing a cancer drug may want to know if it improves life expectancy. A t-test is usually used by drug companies to compare averages. It will then let you know if the probability of those results is happening by chance. A t-test tells us this by sitting side by side the means of two groups. A researcher using this test specifically wants to know if the results are repeatable. It verifies if the differences measured in means could have happened by chance or not. We can conclude that there is a statistically significant relationship between seeing the American flag and being less favourable towards people on welfare.A t-test is used to find significant differences between groups. Although this is a rather small difference in the sample means, the t-test and its associated level of significance lead us to reject the null hypothesis that these two means are equal in the population ( t = 2.82, df = 3062, p-value = 0.005). We see the mean difference between the two groups is about 0.24 of a point on the favourableness towards welfare recipients scale. That means we can focus our remaining attention on the results for t-test when equal variances are assumed. The F-statistic for the LeveneвЂ™s test tells us we cannot reject the null hypothesis that the variance of welfare is equal across the two groups. It is common to reject a null hypothesis when a reported level of statistical significance for a test is less than 0.05. It also produces the t-test, the degrees of freedom, a significance level, the mean difference, the standard error of that mean difference, and a 95% confidence interval for that mean difference under both the assumption that the variances of the two groups are equal and the assumption that they are not.ДЇigure 6: Results from using a t-test to test the difference between treatment and control group opinions about welfare recipients. First it provides an F-test (denoted as LeveneвЂ™s test) of whether or not the variances of welfare for the two groups can be assumed to be equal or not, along with an appropriate estimate of statistical significance. SPSS produces a lot of output in this table. The second table shown in Figure 6 reports the results of the difference of means t-test. welfare) for each of the two groups of observations (e.g. It simply reports summary statistics for the test variable (e.g. The SPSS procedure for conducting a difference of means t-test outlined above produces two tables. Next we compare the mean of our continuous variable for the two groups of participants to see if there is a difference.

The flag variable is dichotomous, representing treatment or control group, making the difference of means t-test appropriate for this example.ДЇigure 4: Frequency distribution of treatment and control groups.ДЇigures 3 and 4 show the distribution of each of these variables by themselves. We treat the welfare variable as continuous. The distribution of the variable is as follows:

The variable welfare is measured on a scale from 1 to 10 where a higher score means more favourable towards welfare recipients. These two values denote whether a respondent was randomly allocated to be shown images of the American flag (treatment) or not (control) before answering the questions about welfare recipients. The first variable, flag, is coded 1 = вЂвЂFlagвЂ™вЂ™ or 0 = вЂвЂNo flagвЂ™вЂ™. A question asking how favourable or unfavourable peopleвЂ™s opinions are about welfare recipients ( welfare).Whether the respondent was exposed to images of the American flag during the experiment ( flag).This example uses a subset of data from TESS2013. 2 An Example in SPSS: Opinions About Welfare Recipients

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