Wikipedia defines this as:

In statistics, the variance inflation factor (VIF) is a method of detecting the severity of multicollinearity. More precisely, the VIF is an index which measures how much the variance of a coefficient (square of the standard deviation) is increased because of collinearity. [Ref]

Here’s an implementation using CenterSpace’s NMath and NMath Stats libraries.

// Returns all the variance inflation factors
private static DoubleVector Vif( LinearRegression lr )
{
  // iterate through predictors and find variance
  // inflation factor for each
  DoubleVector factors =
    new DoubleVector( lr.NumberOfPredictors );
  for (int i = 0; i < lr.NumberOfPredictors; i++)
  {
  factors[i] = Vif( lr, i );
  }
  return factors;
}

// Returns a single variance inflation factor
private static double Vif( LinearRegression lr, int i )
{
  // remove predictor, change observation
  LinearRegression lr2 = (LinearRegression)lr.Clone();
  lr2.RemovePredictor( i );
  lr2.SetRegressionData( lr2.PredictorMatrix,
    lr.PredictorMatrix.Col( i ), true );

  // calculate variance inflation factor
  LinearRegressionAnova anova =
    new LinearRegressionAnova( lr2 );

  // return factor
  return 1.0 / (1.0 - anova.RSquared);
}

And here’s an example using these functions:

DoubleMatrix independent = new DoubleMatrix(
  "30x3[0.270 78 41 0.282 79 56 0.277 81 63 " +
       "0.280 80 68 0.272 76 69 0.262 78 65 " +
       "0.275 82 61 0.267 79 47 0.265 76 32 " +
       "0.277 79 24 0.282 82 28 0.270 85 26 " +
       "0.272 86 32 0.287 83 40 0.277 84 55 " +
       "0.287 82 63 0.280 80 72 0.277 78 72 " +
       "0.277 84 67 0.277 86 60 0.292 85 44 " +
       "0.287 87 40 0.277 94 32 0.285 92 27 " +
       "0.282 95 28 0.265 96 33 0.265 94 41 " +
       "0.265 96 52 0.268 91 64 0.260 90 71]" );

DoubleVector dependent =
  new DoubleVector( "0.386 0.374 0.393 0.425 " +
  "0.406 0.344 0.327 0.288 0.269 0.256 0.286 " +
  "0.298 0.329 0.318 0.381 0.381 0.470 0.443 " +
  "0.386 0.342 0.319 0.307 0.284 0.326 0.309 " +
  "0.359 0.376 0.416 0.437 0.548" );

LinearRegression regression =
  new LinearRegression( independent, dependent, true );
Console.WriteLine( "Is good? " + regression.IsGood );
LinearRegressionAnova anova =
  new LinearRegressionAnova( regression );
Console.WriteLine( "variance: " + regression.Variance );
Console.WriteLine( "r-squared: " + anova.RSquared );
DoubleVector vif = Vif( regression );
Console.WriteLine( "variance inflation factors: " + vif );

-Trevor

Note: This functionality is now in NMath Stats.

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