Prof Brian Ripley wrote: > The derivative of plogis is surely dlogis. (And yes, there is a good > reason why we have such a function: take a look at its C code.)
Doh. > > That means we would need an entry for dlogis too, I guess. I am not > convinced that there is a real need for these (and where does this > stop?) What would be much more useful is to make this user-extensible > (as Bill Venables pointed out a decade ago). [pd]norm were added in > 2002 to support MASS, the ability to do all of MASS in R being a goal at > the time. I agree it would be great to make this user-extensible, but it's probably a bit beyond me ... I found a web-reference of Venables saying > There is a detailed example towards the end of Ch. 9 of V&R on how > to extend D() and make.call(), which are the work horses for > deriv(), to handle new functions. The new functions handled there > are dnorm() and pnorm(), but I() would be even easier, of course. http://www.math.yorku.ca/Who/Faculty/Monette/pub/s-old/0690.html ... but this is from 1997 therefore presumably MASS3? or MASS2? -- I can't find my copy of MASS3 at the moment, and don't own MASS2 ... The reason behind this is that I was trying to write a simple analytic derivative calculator for formulae of the form (e.g.) y ~ dbinom(prob=plogis(a+b*x),size=N) Obviously in this case I could just tell people to write the formula out as y ~ dbinom(prob=1/(1+exp(-(a+b*x))),size=N) ... Ben Bolker
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