# [R] Greater than 1 or less than 1?

Zhen Chen zchen at cceb.upenn.edu
Mon Jun 21 00:32:46 CEST 2004

```I have problem evaluating the expression h = exp(x)/(exp(exp(x))-1) for
large negative x. This expression is actually the probability
that y = 1 when y is a Poisson random variable truncated at 0, hence
must satisfy 0 <= h <= 1. However, when
x < -18, I may get an h value that is  larger than 1 while the true
value should be a quantity that is smaller than but very close to 1.
For example when x = -19, h = 1.00000000031174. I tried to use
different form of h and none of
them give me an h value that is less than or equal to 1. I also tried
to find patterns, but discovered none: for some x < -18, h is below 1;
for others
h is greater than 1. Is there any trick that enables me to obtain
theoretically correct results from this expression?

Thanks.

Zhen Chen, Ph.D.
Assistant Professor of Biostatistics
Department of Biostatistics & Epidemiology
University of Pennsylvania School of Medicine
423 Guardian Drive -- 625 Blockley Hall