Hi, 

I'm trying to find a way to take evaluate the Normal CDF over an interval and return the result on the log scale.  This works, but I think it isn't numerically stable:  

log(pnorm(a, mean = x, sd = y) - pnorm(b, mean = x, sd = y))

Does anyone know of a single function that does the above?  Or knows of a way to make it more stable?  I'd really appreciate any suggestions!  

Thanks, 

Olivia
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