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Creating box plots for quantitative variables

From: SPSS Statistics Essential Training (2011)

Video: Creating box plots for quantitative variables

When you're looking at what SPSS calls a scale variable--that's something that can be measured as more or less, like the percentage of critics who gave a favorable rating to a movie or the budget or the box office earnings for that movie--you should generally make two kinds of charts. The first one, which we did in the last movie, is called a histogram. It's like a bell curve, and it's a good way of getting a feel for the overall shape of a distribution. The second kind that you should generally make for a scale variable is called a box plot, and it's primary purpose in this context is to check for outlying scores, because they can cause a lot of problems in later statistical analyses.

Creating box plots for quantitative variables

When you're looking at what SPSS calls a scale variable--that's something that can be measured as more or less, like the percentage of critics who gave a favorable rating to a movie or the budget or the box office earnings for that movie--you should generally make two kinds of charts. The first one, which we did in the last movie, is called a histogram. It's like a bell curve, and it's a good way of getting a feel for the overall shape of a distribution. The second kind that you should generally make for a scale variable is called a box plot, and it's primary purpose in this context is to check for outlying scores, because they can cause a lot of problems in later statistical analyses.

So you need to be able to identify whether you have outliers and often what those outliers are. So what I'm going to do now is I'm going to create a box plot for budget, which we used in the last movie on histograms. Come up to Graphs, to the Chart Builder, and from there I come down to the list, to Boxplot. There are several different versions of box plots. I am going to choose the simplest one possible. That's this one over here, which is called a 1-D Boxplot. It's for charting all of the cases on a single variable.

If I wanted to break down budgets by a genre of film, I could do that over here, under what's called a Simple Boxplot, but it's grouped, and I will show that in a later movie. But right now I'm simply going to drag the 1-D Boxplot up to the canvas, and then I'm going to bring in budget to the Y axis. This is the general format of a box plot. I will explain more when we look at the finished version. But I am going to do a couple of things. Number one is I may want to identify points.

If click on Point ID Label, and then I can actually get the movie name and I can drag that into here, so if I have unusually high or low points, it will actually tell me what the movie is. It makes life easier. I can also put titles on. I will have a title, and I will put Boxplot of Movie Budgets. Then I will press Apply, and for both of these I can now press OK over here. And what comes up is this particular chart. This is the text that is the syntax that produces the command.

This is the name of the command, this is the data set, Movies.sav, and this is the Boxplot of Movie Budgets. What you have here is budgets ranging from 0-- there's actually nothing with 0-- up to $250 million for the movie. This is from a few years ago. And this box right here shows the quartiles of a distribution, and this is the minimum value of any movie in the data set. This right here is the highest non- outlying value, and I say non-outlying because we have two outlier movies.

In this particularly data set Spiderman 2 and King Kong both had budgets of approximately $200 million. On the other hand, this box down here shows you the median, that 50% of the movies--there were 61 in this data set, so 30 of them--had budgets beneath this, which is around $25 or $30 million, and half of them were above. Now, I am going to show you a few ways to modify this chart that I think will make it a little easier to deal with.

As with every chart in SPSS, you modify it by first double-clicking on it to activate it. That brings up the chart in a Chart Editor window and it brings up a Properties window to the right. Now, one thing that I personally like to do is I like to turn these charts sideways by coming up to the button bar and clicking on the button that says "Transpose the chart coordinate system." The reason I do this is because the other charts that we make up, like histograms and like the scatter plots that we will show later, they have these variables listed across the bottom, with the lowest value on the left, highest value on the right, and I find it helpful to be consistent in this particular way.

I'd like to change the color of the chart. I click on the box, come over here to change the fill, and then the border I can change to another color if I want. I can change the way these bars work at the end. These are sometimes called whiskers. They go to the lowest and the highest non-outlying value. In case you're wondering, outliers are determined by being one and a half times of this middle range above or below the range. What we're going to do is I'm going to change the way these whiskers are.

This is just a preference issue. I click on that, and I come over her to Bar Options, and I am going to change it from a T-bar to what's called a Whisker. It's just a line at the end. And then here, if I want to, I can actually change the way that these look at the end. I have the movie labels there as well. Finally, if I want to change the Axis labels here on the bottom, like I did with the histogram where I changed these to millions of dollars, I click on the numbers, and I come over to the Properties window, to Number Format, and the Scaling Factor here, I'm going to put in millions.

I am going to press Apply, and this now gives me millions of dollars. And I need to change this-- it says Budget--to say Budget in Millions. I can close the chart, and now I have a good depiction that the overall distribution is on the low end, because this is movies that included award winners, that half of the movies have budgets of 30 million or less, but they go up to about 150 million, and that in this particular data set we had two other movies--Spiderman 2 and King Kong-- that had unusually large budgets, as is common among summer blockbusters.

Anyhow, when you're looking at a scale variable like budget, like viewer evaluations, like time spent on tasks, like time spent viewing a web site, then you do want to look at both the overall shape of the distribution with the histogram and you want to check for outliers, and a box plot is an ideal way to do that.

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This video is part of

Image for SPSS Statistics Essential Training (2011)
SPSS Statistics Essential Training (2011)

52 video lessons · 20159 viewers

Barton Poulson
Author

 
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  1. 2m 58s
    1. Welcome
      1m 5s
    2. Using the exercise files
      40s
    3. Using a different version of the software
      1m 13s
  2. 19m 0s
    1. Taking a first look at the interface
      11m 49s
    2. Reading data from a spreadsheet
      7m 11s
  3. 21m 54s
    1. Creating bar charts for categorical variables
      7m 18s
    2. Creating pie charts for categorical variables
      2m 54s
    3. Creating histograms for quantitative variables
      5m 45s
    4. Creating box plots for quantitative variables
      5m 57s
  4. 33m 10s
    1. Recoding variables
      5m 33s
    2. Recoding with visual binning
      5m 33s
    3. Recoding by ranking cases
      5m 26s
    4. Computing new variables
      5m 37s
    5. Combining or excluding outliers
      5m 21s
    6. Transforming outliers
      5m 40s
  5. 28m 12s
    1. Selecting cases
      6m 44s
    2. Using the Split File command
      5m 12s
    3. Merging files
      5m 33s
    4. Using the Multiple Response command
      10m 43s
  6. 22m 14s
    1. Calculating frequencies
      8m 43s
    2. Calculating descriptives
      5m 31s
    3. Using the Explore command
      8m 0s
  7. 16m 3s
    1. Calculating inferential statistics for a single proportion
      6m 6s
    2. Calculating inferential statistics for a single mean
      5m 39s
    3. Calculating inferential statistics for a single categorical variable
      4m 18s
  8. 30m 43s
    1. Creating clustered bar charts
      7m 10s
    2. Creating scatterplots
      5m 8s
    3. Creating time series
      3m 24s
    4. Creating simple bar charts of group means
      4m 17s
    5. Creating population pyramids
      3m 0s
    6. Creating simple boxplots for groups
      3m 3s
    7. Creating side-by-side boxplots
      4m 41s
  9. 45m 28s
    1. Calculating correlations
      8m 17s
    2. Computing a bivariate regression
      6m 27s
    3. Creating crosstabs for categorical variables
      6m 34s
    4. Comparing means with the Means procedure
      6m 33s
    5. Comparing means with the t-test
      6m 4s
    6. Comparing means with a one-way ANOVA
      6m 30s
    7. Comparing paired means
      5m 3s
  10. 24m 30s
    1. Creating clustered bar charts for frequencies
      6m 34s
    2. Creating clustered bar charts for means
      3m 45s
    3. Creating scatterplots by group
      4m 13s
    4. Creating 3-D scatterplots
      4m 25s
    5. Creating scatterplot matrices
      5m 33s
  11. 30m 57s
    1. Using Automatic Linear Models
      11m 52s
    2. Calculating multiple regression
      9m 3s
    3. Comparing means with a two-factor ANOVA
      10m 2s
  12. 29m 29s
    1. Formatting descriptive statistics
      6m 1s
    2. Formatting correlations
      7m 49s
    3. Formatting regression
      10m 19s
    4. Exporting charts and tables
      5m 20s
  13. 51s
    1. What's next
      51s

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