asa091, a C code which computes the percentage points of the Chi-Squared probability density function (PDF), by Best and Roberts.

This is Applied Statistics Algorithm 91.


The computer code and data files described and made available on this web page are distributed under the MIT license


asa091 is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version.

Related Data and Programs:


normal, a C code which samples the normal distribution.

prob, a C code which evaluates and inverts many probability distibutions.

test_values, a C code which contains sample values for a number of distributions.

uniform, a C code which samples the uniform distribution.


Original FORTRAN77 version by Donald Best and DE Roberts; C version by John Burkardt.


  1. Donald Best, DE Roberts,
    Algorithm AS 91: The Percentage Points of the Chi-Squared Distribution,
    Applied Statistics,
    Volume 24, Number 3, 1975, pages 385-390.
  2. William Cody, Kenneth Hillstrom,
    Chebyshev Approximations for the Natural Logarithm of the Gamma Function,
    Mathematics of Computation,
    Volume 21, Number 98, April 1967, pages 198-203.
  3. John Hart, Ward Cheney, Charles Lawson, Hans Maehly, Charles Mesztenyi, John Rice, Henry Thacher, Christoph Witzgall,
    Computer Approximations,
    Wiley, 1968,
    LC: QA297.C64.
  4. BL Shea,
    Algorithm AS 239: Chi-squared and Incomplete Gamma Integral,
    Applied Statistics,
    Volume 37, Number 3, 1988, pages 466-473.
  5. Michael Wichura,
    Algorithm AS 241: The Percentage Points of the Normal Distribution,
    Applied Statistics,
    Volume 37, Number 3, 1988, pages 477-484.

Source Code:

Last revised on 19 June 2019.