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Calculating inferential statistics for a single categorical variable

From: SPSS Statistics Essential Training (2011)

Video: Calculating inferential statistics for a single categorical variable

In the last two movies we've looked at the most basic inferential statistics, the ones where we analyzed one variable at a time. We looked at the proportion for a nominal variable, with only two outcomes, that is, a dichotomous variable, and we looked at the mean for a scale variable. In both cases, we looked at both null hypothesis tests and confidence intervals. In this movie, we will expand things slightly by looking at how to do a hypothesis test for a nominal variable, or a categorical variable, that has more than two categories, something like occupation or a favorite sport.

Calculating inferential statistics for a single categorical variable

In the last two movies we've looked at the most basic inferential statistics, the ones where we analyzed one variable at a time. We looked at the proportion for a nominal variable, with only two outcomes, that is, a dichotomous variable, and we looked at the mean for a scale variable. In both cases, we looked at both null hypothesis tests and confidence intervals. In this movie, we will expand things slightly by looking at how to do a hypothesis test for a nominal variable, or a categorical variable, that has more than two categories, something like occupation or a favorite sport.

Although it's possible to do confidence intervals for the number of people in each category, it's a complicated procedure, and it's not particularly helpful for most purposes. Instead, we'll just do a hypothesis test that looks at whether people are evenly distributed across all the categories in the variable. The test statistics that we'll use is called the One Sample Chi-Square Test in SPSS. It's also known as the Goodness-of- fit Test, and with SPSS's new automatic features, this is very easy to create and interpret.

I am going to be using the same data set as before, GSS.sav from the General Social Survey, and I thought it might be interesting to look at the variable that is second from the last, about people feeling happy. Specifically the question is self-rated happiness. Well, we have three possible answers: Not Too Happy, Pretty Happy, and Very Happy. And we can use this test to see if people fall evenly into those three different categories. To do this, we'd go to the Analyze menu, and then down to Nonparametric Tests, and again to One Sample.

This is the same one that we used for the single proportion. We're just going to be doing it a little bit differently this time. I need to go to the Fields tab, and then I have all of the variables that it can test in the Test Field thing. I don't want all of them there. It will be too much output. So what I am going to do is I am going to select all of these and put all of them back, and then I'll bring back over the only one that I want, which is near the bottom of the list, and it's Self-Rated Happiness. I can double-click on that to move it over.

Then I can go with the default test. All I need to do now is press Run, and I get the same kind of table I got before. It lets me know that the null hypothesis, or that the categories of Self-Rated Happiness, would occur with equal probabilities. That is that we would have the same percentage of people who said that they were Not Too Happy and Pretty Happy and Very Happy. All I can tell from this one is that those three are not evenly distributed. But this is an interactive model viewer, so I double-click on it and I will maximize that window.

And what I see is the hypothesized value is the green bars over here, and what it is, you see all three of them are the same size. The blue is how many I actually have. The green is how many I would have expected if things were distributed evenly. And it tells me that I have an observed 43 people who said they were not too happy. That's this blue bar right here, that's the Observed. The hypothesized was 116. So the difference between the two, the residual, is 73. In fact, what you can see is that in the first set, the Not Too Happy, I have fewer people than I would expect if people were evenly distributed.

On the other hand, I have a lot more people in the middle set, Pretty Happy, than I would expect. The Very Happy is actually right around one third of the group. Down below that, I have a table that gives me the total sample size, 349. The Test Statistic there is called the Chi-Square Test, and it's got a value of 88.06. It has what's called 2 Degrees of Freedom, and a Probability value, that's the Asymptotic Significance 2-sided test of less than 000. Again, it's not exactly 0, but it's going to be a small number.

Anyhow, this is the easiest possible hypothesis test for a categorical variable that has several categories in it. The One Sample Chi-Square Test, it's a quick and easy way to tell if your observations are distributed evenly across categories, or you can also specify some other expected way. It shows how important it can be to check whether the variation you see could be reasonably attributed to random, meaning less chance, or whether you might start to see something important that deserves further analysis.

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This video is part of

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

52 video lessons · 20094 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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