This page shows how to perform a number of statistical tests using SPSS. Each section gives a brief description of the aim of the statistical test, when it is used, an example showing the SPSS commands and SPSS (often abbreviated) output with a brief interpretation of the output. You can see the page Choosing the Correct Statistical Test for a table that shows an overview of when each test is appropriate to use. In deciding which test is appropriate to use, it is important to consider the type of variables that you have (i. e.
, whether your variables are categorical, ordinal or interval and whether they are normally distributed), see What is the difference between categorical, ordinal and interval variables? for more information on this. About the hsb data file Most of the examples in this page will use a data file called hsb2, high school and beyond. This data file contains 200 observations from a sample of high school students with demographic information about the students, such as their gender (female), socio-economic status (ses) and ethnic background (race).
It also contains a number of scores on standardized tests, including tests of reading (read), writing (write), mathematics (math) and social studies (socst). You can get the hsb data file by clicking on hsb2. One sample t-test A one sample t-test allows us to test whether a sample mean (of a normally distributed interval variable) significantly differs from a hypothesized value. For example, using the hsb2 data file, say we wish to test whether the average writing score (write) differs significantly from 50. We can do this as shown below. t-test /testval = 50 /variable = write.
The mean of the variable write for this particular sample of students is 52. 775, which is statistically significantly different from the test value of 50. We would conclude that this group of students has a significantly higher mean on the writing test than 50. One sample median test A one sample median test allows us to test whether a sample median differs significantly from a hypothesized value. We will use the same variable, write, as we did in the one sample t-test example above, but we do not need to assume that it is interval and normally distributed (we only need to assume that write is an ordinal variable).
nptests /onesample test (write) wilcoxon(testvalue = 50). 06/14/2012 11:32 AM Statistical Tests in SPSS 2 of 27 http://www. ats. ucla. edu/stat/spss/whatstat/whatstat. htm Binomial test A one sample binomial test allows us to test whether the proportion of successes on a two-level categorical dependent variable significantly differs from a hypothesized value. For example, using the hsb2 data file, say we wish to test whether the proportion of females (female) differs significantly from 50%, i. e. , from . 5. We can do this as shown below. npar tests /binomial (. 5) = female.
The results indicate that there is no statistically significant difference (p = . 229). In other words, the proportion of females in this sample does not significantly differ from the hypothesized value of 50%. Chi-square goodness of fit A chi-square goodness of fit test allows us to test whether the observed proportions for a categorical variable differ from hypothesized proportions. For example, let’s suppose that we believe that the general population consists of 10% Hispanic, 10% 06/14/2012 11:32 AM Statistical Tests in SPSS 3 of 27 http://www. ats. ucla. edu/stat/spss/whatstat/whatstat. htm
Asian, 10% African American and 70% White folks. We want to test whether the observed proportions from our sample differ significantly from these hypothesized proportions. npar test /chisquare = race /expected = 10 10 10 70. These results show that racial composition in our sample does not differ significantly from the hypothesized values that we supplied (chi-square with three degrees of freedom = 5. 029, p = . 170). Two independent samples t-test An independent samples t-test is used when you want to compare the means of a normally distributed interval dependent variable for two independent groups.
For example, using the hsb2 data file, say we wish to test whether the mean for write is the same for males and females. t-test groups = female(0 1) /variables = write. Because the standard deviations for the two groups are similar (10. 3 and 8. 1), we will use the “equal variances assumed” test. The results indicate that there is a statistically significant difference between the mean writing score for males and females (t = -3. 734, p = . 000). In other words, females have a statistically significantly higher mean score on writing (54. 99) than males (50. 12). See also
SPSS Learning Module: An overview of statistical tests in SPSS Wilcoxon-Mann-Whitney test 06/14/2012 11:32 AM Statistical Tests in SPSS 4 of 27 http://www. ats. ucla. edu/stat/spss/whatstat/whatstat. htm The Wilcoxon-Mann-Whitney test is a non-parametric analog to the independent samples t-test and can be used when you do not assume that the dependent variable is a normally distributed interval variable (you only assume that the variable is at least ordinal). You will notice that the SPSS syntax for the Wilcoxon-Mann-Whitney test is almost identical to that of the independent samples t-test.
We will use the same data file (the hsb2 data file) and the same variables in this example as we did in the independent t-test example above and will not assume that write, our dependent variable, is normally distributed. npar test /m-w = write by female(0 1). The results suggest that there is a statistically significant difference between the underlying distributions of the write scores of males and the write scores of females (z = -3. 329, p = 0. 001). See also FAQ: Why is the Mann-Whitney significant when the medians are equal? Chi-square test
A chi-square test is used when you want to see if there is a relationship between two categorical variables. In SPSS, the chisq option is used on the statistics subcommand of the crosstabs command to obtain the test statistic and its associated p-value. Using the hsb2 data file, let’s see if there is a relationship between the type of school attended (schtyp) and students’ gender (female). Remember that the chi-square test assumes that the expected value for each cell is five or higher. This assumption is easily met in the examples below. However, if this assumption is not met in your data, please see the section on Fisher’s exact test below.