gaussian family change suggestion

On Tue, 11 Apr 2006, Simon Wood wrote:

Hi,

Currently the `gaussian' family's initialization code signals an error if
any response data are zero or negative and a log link is used. Given that
zero or negative response data are perfectly legitimate under the GLM
fitted using `gaussian("log")', this seems a bit unsatisfactory. Might
it be worth changing it?

Well, that's not really true: it says it is unable to find starting
values, and asks you to provide some.  I don't really see why your choices
are satisfactory: what happens if all the data are negative for example?
(You get +Inf in the log case, I believe.  The algorithm will probably get
stuck from a finite starting point, but it will produce a mysterious error
from an infinite one.)

We could try even harder, but code that is almost never used tends to get
broken whilst no one is testing it.  So if you want to pursue it I think
we need a comprehensive test suite.

The current offending code from `gaussian' is:

initialize = expression({
           n <- rep.int(1, nobs)
           if (is.null(etastart) && is.null(start) && is.null(mustart) &&
               ((family$link == "inverse" && any(y == 0)) ||
                 (family$link == "log" && any(y <= 0)))) stop(
             "cannot find valid starting values: please specify some")
           mustart <- y
       })

A possible replacement would be ...

initialize = expression({
           n <- rep.int(1, nobs)
           if (is.null(etastart) && is.null(start) && is.null(mustart) &&
               ((family$link == "inverse" && all(y == 0)) ||
                 (family$link == "log" && all(y <= 0)))) stop(
     "cannot find valid starting values: please specify some")
       mustart <- y
     if (family$link=="log") {
       iy <- y<=0
       if (sum(iy)) mustart[iy] <- min(y[!iy])*.5
     } else if (family$link=="inverse") {
       iy <- y==0
       if (sum(iy)) mustart[iy] <- min(abs(y[!iy]))*.5
     }
       })

best,
Simon

- Simon Wood, Mathematical Sciences, University of Bath, Bath BA2 7AY
-             +44 (0)1225 386603         www.maths.bath.ac.uk/~sw283/

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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