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Lesson 3 in SPSS How to find measures variability using SPSS The Dataset • Here’s a nice dataset. • We have one variable called Age. • There are 1,514 observations in the dataset. First Blush • To get a quick picture of this dataset, let’s see a frequency distribution histogram (Lesson 1). • Hmm, perhaps a bit skewed? Selecting the Analysis • From the SPSS menu bar, choose • Analyze • Descriptive statistics • Frequencies Select the Variable(s) • In the Frequencies box, highlight the variable age, then click on the arrow to pop it into the Variables window. Descriptives Box • Notice that when you’ve done this, the OK box is now active. • But let’s make sure we get the statistics we want. Selecting the Statistics • I’ve selected the mean, median and mode as my measures of central tendency. Plus, I asked for the sum. • For my measures of spread, I’ve chosen standard deviation, variance, and range. Plus I asked for the minimum and maximum values. The Interquartile Range • To find the interquartile range in SPSS, select Quartiles. • I’ve also asked it for a measure of the skewness of the distribution. • Now click on Continue. Running the Analysis • Now we can click on OK. The Output • • • • So what did we learn? The mode is 35, the median is 41.00, and the mean is 45.63. These measures appear to be the perfect definition of a positively skewed distribution. The range is 71 and goes from a minimum of 18 years to a maximum of 89 years old. The sample variance is 317.14 and taking the square root of that we have the sample standard deviation of 17.81 Statistics Age of Res pondent N Mean Median Mode Std. Deviation Variance Skewness Std. Error of Skewness Range Minimum Maximum Sum Percentiles Valid Missing 25 50 75 1514 3 45.63 41.00 35 17.808 317.140 .524 .063 71 18 89 69078 32.00 41.00 60.00 More Output • To find the interquartile range, we take the 75th percentile minus the 25th percentile. Here, it is 60 – 32 = 28. So the SIQ = 28/2 = 14. • Also, we note our skewness value is .524 with a standard error of .063. Don’t worry about that now, we’ll look at this again in Lesson 4. Statistics Age of Res pondent N Mean Median Mode Std. Deviation Variance Skewness Std. Error of Skewness Range Minimum Maximum Sum Percentiles Valid Missing 25 50 75 1514 3 45.63 41.00 35 17.808 317.140 .524 .063 71 18 89 69078 32.00 41.00 60.00 Visual Representation Median Mode • Let’s mark these on our graph. Mean Mean SIQ = 14 s = 17.81 Range = 71