The centre of distribution C is the highest of the three distributions (median is 0.88).It’s the most concentrated distribution because the interquartile range is 0.21, compared to 0.30 for distribution A and 0.26 for distribution C. Distribution B is approximately symmetric, because both half-boxes are almost the same length (0.11 on the left side and 0.10 on the right side).The distribution is positively skewed, because the whisker and half-box are longer on the right side of the median than on the left side. The centre of distribution A is the lowest of the three distributions (median is 0.11).The information is grouped by Measurement (appearing as row headers), Distribution A, Distribution B and Distribution C (appearing as column headers). This table displays the results of Data table for chart 4.5.2.1. Example 1 – Comparison of three box and whisker plots Data points that are outside this interval are represented as points on the graph and considered potential outliers. That is, the whisker reaches the value that is the furthest from the centre while still being inside a distance of 1.5 times the interquartile range from the lower or upper quartile. The box and whisker plot can be presented horizontally, like in figure 4.5.2.1, or vertically.Ī variation of the box and whisker plot restricts the length of the whiskers to a maximum of 1.5 times the interquartile range.The graph is usually presented with an axis that indicates the values (not shown on figure 4.5.2.1).The whiskers are the two lines outside the box, that go from the minimum to the lower quartile (the start of the box) and then from the upper quartile (the end of the box) to the maximum.Sometimes, the mean is also indicated by a dot or a cross on the box plot. The vertical line that split the box in two is the median.The box covers the interquartile interval, where 50% of the data is found. The left and right sides of the box are the lower and upper quartiles.
The figure shows the shape of a box and whisker plot and the position of the minimum, lower quartile, median, upper quartile and maximum.
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A box plot is ideal for comparing distributions because the centre, spread and overall range are immediately apparent.įigure 4.5.2.1 shows how to build the box and whisker plot from the five-number summary. It doesn’t show the distribution in as much detail as histogram does, but it’s especially useful for indicating whether a distribution is skewed and whether there are potential unusual observations (outliers) in the data set. You should now be familiar with one method for calculating the mean and standard deviation within the SPSS statistics package.Ĭheck out our YouTube channel for video tutorials, including this tutorial that shows you step-by-step how to calculate the mean and standard deviation in SPSS.The box and whisker plot, sometimes simply called the box plot, is a type of graph that help visualize the five-number summary. The SPSS output viewer will appear with the following result (though, of course, the result will be different according to the data you enter).Īs you can see, the values for the mean and standard deviation appear next to the value for N (which is the number of items in your dataset). Press Continue, and then press OK in the Descriptives dialog box. deviation (and if you wish you can choose additional measures – e.g., Skewness). Now choose Options, and select Mean and Std. This can be done by selecting it on the left, and then clicking the blue arrow button. You need to get the variable for which you want to know the mean and standard deviation into the variables box on the right (as per the image above). This will open up the following dialog box.
To calculate the mean and standard deviation, choose Analyze -> Descriptive Statistics -> Descriptives, as below. In other words, you want to know the average time it took to do the task, and how much the times vary – their spread. You want to find out the mean and standard deviation of the duration variable. We’re working on the assumption that you have already imported your data into SPSS, and you’re looking at something a bit like this (though obviously with different variables, figures, etc).Īs you can see, we’ve got three variables: (a) duration – which is the duration in seconds it takes to complete a certain task (b) sex – male or female and (c) height – in inches.
There are a number of different ways to achieve this, but what follows is probably the easiest. Result will appear in the SPSS output viewer.Click Options, and select Mean and Standard Deviation.Drag the variable of interest from the left into the Variables box on the right.Click Analyze -> Descriptive Statistics -> Descriptives.