# Homogeneity Of Variances Tests QuickStart Sample (Visual Basic)

Illustrates how to test a collection of variables for equal variances using classes in the Extreme.Statistics.Tests namespace in Visual Basic.

View this sample in: C# F# IronPython

```
Option Infer On
Imports Extreme.Mathematics
Imports Extreme.Statistics
Imports Extreme.Statistics.Tests
' Illustrates hypothesis tests for the homogeneity of variances
' using classes from the Extreme.Statistics.Tests namespace.
Module HomogeneityOfVariancesTests
Sub Main()
' The license is verified at runtime. We're using
' a demo license here. For more information, see
' https://www.extremeoptimization.com/trial-key
Extreme.License.Verify("Demo license")
' One of the underlying assumptions of Analysis of Variance
' (ANOVA) is that the variances in the different groups are
' identical. This QuickStart Sample shows how to use
' the two tests are available that can verify this assumption.
' The data for this QuickStart Sample is measurements of
' the diameters of gears from 10 different batches.
' Two variables are provided:
' batchVariable contains the batch number of each measurement:
Dim batch = Vector.Create(New Integer() _
{
1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
9, 9, 9, 9, 9, 9, 9, 9, 9, 9,
10, 10, 10, 10, 10, 10, 10, 10, 10, 10
}).AsCategorical()
' diameterVariable contains the actual measurements:
Dim diameter = Vector.Create(New Double() _
{
1.006, 0.996, 0.998, 1.0, 0.992, 0.993, 1.002, 0.999, 0.994, 1.0,
0.998, 1.006, 1.0, 1.002, 0.997, 0.998, 0.996, 1.0, 1.006, 0.988,
0.991, 0.987, 0.997, 0.999, 0.995, 0.994, 1.0, 0.999, 0.996, 0.996,
1.005, 1.002, 0.994, 1.0, 0.995, 0.994, 0.998, 0.996, 1.002, 0.996,
0.998, 0.998, 0.982, 0.99, 1.002, 0.984, 0.996, 0.993, 0.98, 0.996,
1.009, 1.013, 1.009, 0.997, 0.988, 1.002, 0.995, 0.998, 0.981, 0.996,
0.99, 1.004, 0.996, 1.001, 0.998, 1.0, 1.018, 1.01, 0.996, 1.002,
0.998, 1.0, 1.006, 1.0, 1.002, 0.996, 0.998, 0.996, 1.002, 1.006,
1.002, 0.998, 0.996, 0.995, 0.996, 1.004, 1.004, 0.998, 0.999, 0.991,
0.991, 0.995, 0.984, 0.994, 0.997, 0.997, 0.991, 0.998, 1.004, 0.997
})
' To prepare the data, we create a vector of vectors,
' one for each batch. This is optional. (See below.)
Dim variables = diameter.SplitBy(batch)
'
' Bartlett's test
'
' Bartlett's test is relatively fast, but has the drawback that
' it requires the data in the groups to be normally distributed,
' and it is not very robust against departures from normality.
' What this means in practice is that the test can't distinguish
' between rejection because of non-homogeneity of variances
' and violation of the normality assumption.
Console.WriteLine("Bartlett's test.")
' We pass the array of variables to the constructor:
Dim bartlett As New BartlettTest(variables)
' We could have also written
Dim bartlett2 = New BartlettTest(diameter, batch)
' We can obtain the value of the test statistic through the Statistic property,
' and the corresponding P-value through the Probability property:
Console.WriteLine("Test statistic: {0:F4}", bartlett.Statistic)
Console.WriteLine("P-value: {0:F4}", bartlett.PValue)
Console.WriteLine("Critical value: {0:F4} at 90%",
bartlett.GetUpperCriticalValue(0.1))
Console.WriteLine("Critical value: {0:F4} at 95%",
bartlett.GetUpperCriticalValue(0.05))
Console.WriteLine("Critical value: {0:F4} at 99%",
bartlett.GetUpperCriticalValue(0.01))
' We can now print the test results:
Console.WriteLine("Reject null hypothesis? {0}",
If(bartlett.Reject(), "yes", "no"))
'
' Levene's Test
'
' Levene's test is slower than Bartlett's test, but is generally more reliable.
' It comes in three variants, depending on the measure of location used.
' The default is that the group median is used.
Console.WriteLine()
Console.WriteLine("Levene's Test")
' Once again, we pass an array of Variable objects to the constructor.
' The LeveneTest constructor is overloaded: you can specify
' the type of mean (mean, median, or trimmed mean):
Dim levene As LeveneTest = New LeveneTest(diameter, batch, LeveneTestLocationMeasure.Median)
' We can obtain the value of the test statistic through the Statistic property,
' and the corresponding P-value through the Probability property:
Console.WriteLine("Test statistic: {0:F4}", levene.Statistic)
Console.WriteLine("P-value: {0:F4}", levene.PValue)
' We can obtain critical values for various significance levels:
Console.WriteLine("Critical value: {0:F4} at 90%",
levene.GetUpperCriticalValue(0.1))
Console.WriteLine("Critical value: {0:F4} at 95%",
levene.GetUpperCriticalValue(0.05))
Console.WriteLine("Critical value: {0:F4} at 99%",
levene.GetUpperCriticalValue(0.01))
' We can now print the test results:
Console.WriteLine("Reject null hypothesis? {0}",
If(levene.Reject(), "yes", "no"))
Console.WriteLine("Press Enter key to continue.")
Console.ReadLine()
End Sub
End Module
```