Using SPSS and PASW/Confidence Intervals

Confidence Intervals help us estimate parameters of a population from sample statistics. Confidence Intervals are a range of possible values of a parameter. We express this interval with a specific degree of confidence. The degree of confidence tells a reader how confident we are that the population parameter falls within our stated interval.

For this example we will use the variable “How Spiritual do you consider yourself?” in our example data set made up of genetic counselors. The name of this variable in the data set is “sprscale.” This variable allowed the sample of genetic counselors to rank how spiritual they are on a scale of 1 to 10.

To compute a confidence interval in SPSS, you begin by selecting “Analyze” → “Descriptive Statistics” → “Explore.” Once the “Explore” window pops up, scroll down on the left list until you find “sprscale,” then click on the arrow that will send it to the dependent list:

16confidenceintervals01.png

Now, click on “Statistics”:

16confidenceintervals02.png

Check the box “Descriptives” and type in the level of confidence that you would like to use. For this example we'll use 95%. Click “Continue” then “OK.” The Output Window will now pop up with a variety of statistics, including the confidence interval:

16confidenceintervals03.png

The confidence interval we found for how spiritual Genetic Counselors are on a scale of 1 to 10 is 5.99 to 6.36. We interpret this in plain language by saying “We are 95% confident that the true mean spirituality on a scale of 1 to 10 for the population of genetic counselors is between 5.99 and 6.36.”

Chapter contributed by Lauren Takemoto.

Last modified on 29 June 2010, at 02:59