help centcalc                                                   Patrick Royston
-------------------------------------------------------------------------------

Centile calculation


Syntax


        centcalc mvar svar [, options ]


    options               Description
    -------------------------------------------------------------------------
      centiles(numlist)   defines the required centiles of yvar|xvar
      dist(dist_name)     specifies the distribution of the response variable
      gamma(# | gvar)     G
      delta(# | dvar)     D
      cv                  produces estimated parameters and centiles at all
                            non-missing values of xvar
      prefix(stub)        defines a censoring variable
    -------------------------------------------------------------------------


Description

    centcalc calculates distribution-based centiles from user-supplied
    parameter values for location (mvar), scale (svar) and shape (gamma() and
    delta()). The distributions (dist()) are all transformations of the
    Normal (Gaussian) distribution.


Options

    centiles(numlist) is the list of estimated centiles (e.g. 50 for the
        median) to be calculated.

    cv is an option relating to the parameterisation of the S-curve for the
        chosen distribution.  See xriml for more information.

    delta(# | dvar) defines the (second) shape parameter as a variable (dvar)
        or as a number (#).  If delta() is not specified, it is taken from
        the macro $S_delta.

    dist(dist_name) determines the distributional model (see xriml and
        Remarks below for further details). dist_name must be one of n, en,
        men, pn, mpn, or sl.  If dist() is not specified, dist_name is taken
        from the macro $S_dist.

    gamma(# | gvar) defines the (first) shape parameter as a variable (gvar)
        or as a number (#).  If gamma() is not specified, it is taken from
        the macro $S_gamma.

    prefix(stub) is the prefix of the names of new variables created to hold
        the estimated centiles, each labelled appropriately.  Default stub is
        C.


Remarks

    Further details of the distributions:

    --------------------------------------------------------------------------
    Code     Name of distribution   Location   Scale           Shape param(s)
    --------------------------------------------------------------------------
    n        Normal (Gaussian)      mean       std. dev.       (not applic.)
    en       Exponential-Normal     median     std. dev.-like  skewness-like
    men      Modulus exp-Normal     median     std. dev.-like  (see below)
    pn       Power-Normal           median     std. dev.-like  power transf.
    mpn      Modulus pow-Normal     median     std. dev.-like  (see below)
    sl       Shifted lognormal      median     std. dev.-like  skewness-like
    --------------------------------------------------------------------------

    The mpn and men distributions each have two shape parameters, the first
    (denoted by G) related to skewness and the second (denoted by D) related
    to kurtosis.


Examples

    . centcalc M_ml S_ml, dist(n) centiles(10 90)

    . centcalc M_ml S_ml, dist(en) centiles(10 90) gamma(G_ml)

    . centcalc median cv, dist(pn) centiles(3 25 75 97) gamma(lambda) cv

    . centcalc M S, dist(men) centiles(1 3 50 97 99) gamma(g) delta(d)


Author

    Patrick Royston, MRC Clinical Trials Unit, London.
    patrick.royston@ctu.mrc.ac.uk

    Eileen Wright, Macclesfield


Also see

    Manual:  [R] fracpoly

    Online:  xriml, xrigls (if installed), fracpoly