/*****************************************************************************
*                                                                            *
*   A2STD.SPL   Copyright (C) 2000-2001 DSP Development Corporation          *
*                               All Rights Reserved                          *
*                                                                            *
*   Author:      Randy Race                                                  *
*                                                                            *
*   Synopsis:    Converts an alpha confidence level to standard deviations   *
*                                                                            *
*   Revisions:    3 Apr 2000  RRR  Creation                                  *
*                 7 Dec 2001  RRR  Use better INVPROBN function              *
*                                                                            *
*****************************************************************************/

#if @HELP_A2STD

    A2STD

    Purpose: Converts an alpha confidence level to a standard deviation range

    Syntax:  A2STD(alpha)

              alpha - real or series, the confidence level(s), defaults
                      to, 0.01, a 99% confidence level.


    Returns: a real or series, standard deviation range for the
             given confidence level(s).


    Example:
             a2std(0.01)

             returns 2.5758 the standard deviation range for a
             confidence level of 99%


    Example:
             W1: {0.01, 0.02, 0.03, 0.04, 0.05}
             W2: a2std(w1)

             W2 == {2.5758, 2.3263, 2.1701, 2.0537, 1.9600}

             the standard deviation ranges for confidence levels
             99%, 98%, 97%, 96%, and 95%

    Remarks:
             A2STD uses the built-in INVPROBN function to lookup a Z value
             for a given probability.

    See Also:
             Invprobn
#endif


/* alpha to std range */
a2std(a)
{
        if (argc < 1) a = 0.01;

        return(invprobn(.5 * (2 - a)));
}