function variance = negative_binomial_variance ( a, b )

%*****************************************************************************80
%
%% negative_binomial_variance() returns the variance of the Negative Binomial PDF.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    19 September 2004
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    integer A, a parameter of the PDF.
%    0 <= A.
%
%    real B, a parameter of the PDF.
%    0 < B <= 1.
%
%  Output:
%
%    real VARIANCE, the variance of the PDF.
%
  variance = a * ( 1.0 - b ) / ( b * b );

  return
end
