AP Statistics Curriculum 2007 NonParam VarIndep
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General Advance-Placement (AP) Statistics Curriculum - Variances of Two Independent Samples
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Differences of Variances of Independent Samples
It is frequently necessary to test if k samples have equal variances. Homogeneity of variances is often a reference to equal variances across samples. Some statistical tests, for example the analysis of variance, assume that variances are equal across groups or samples.
Approach
The (modified) Fligner-Killeen test provides the means for studying the homogeneity of variances of k populations { X_{i,j}, for and }. The test jointly ranks the absolute values of and assigns increasing scores , based on the ranks of all observations, see the Conover, Johnson, and Johnson (1981) reference below.
In this test, is the sample median of the j^{th} population, and Φ(.) is the cumulative distribution function for Normal distribution. The Fligner-Killeen test is sometimes also called the median-centering Fligner-Killeen test.
- Fligner-Killeen test statistics:
- ,
- where is the mean score for the j^{th} sample, is the overall mean score of all a_{N,i}, and V^{2} is the sample variance of all scores.
That is:
- ,
- , where is the increasing rank score for the i^{th}-observation in the j^{th}-sample,
- ,
- .
- Fligner-Killeen probabilities:
For large sample sizes, the modified Fligner-Killeen test statistic has an asymptotic chi-square distribution with (k-1) degrees of freedom
- .
- Note:
- Conover, Johnson, and Johnson (1981) carried a simulation comparing different variance homogeneity tests and reported that the modified Fligner-Killeen test is most robust against departures from normality.
Computational Resources: Internet-based SOCR Tools
Still under development.
Examples
Suppose we wanted to study whether the variances in certain time period (e.g., 1981 to 2006) of the consumer-price-indices (CPI) of several items were significantly different. We can use the SOCR CPI Dataset to answer this question for the Fuel, Oil, Bananas, Tomatoes, Orange Juice, Beef and Gasoline items.
Alternative tests of Variance Homogeneity
References
- Conover, W. J., Johnson, M.E., and Johnson M. M. (1981), A comparative study of tests for homogeneity of variances, with applications to the outer continental shelf bidding data. Technometrics 23, 351-361.
- SOCR Home page: http://www.socr.ucla.edu
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