<p>Poisson. I was working nonparametrically. You could use a Poisson model if you wanted to. Then it would be 1-exp(-2). But it requires extra assumptions.</p>
<p>Square root of the mean???? What funny stats book have you been using?</p>
<p>P(X=x) = (exp(-lambda) lambda^x)/x! </p>
<p>is the usual parametrization of the Poisson distribution, with E(X)=Var(X)=lambda. If the sample mean and variance of a large sample are not approximately equal, you have to deal with over/underdispersion in your model...</p>
<p>/I would go for the nonparametric estimate, but your instructor probably won't appreciate. I also don't see why you'd have to express 1-exp(-2) in decimal notation.</p>
<p>Mean and standard deviation of a poisson random variable with parameter lamda are mean = lambda and standard deviation = sqrt lambda respectively. Thats all the book I have says and in the problems it just gave lambda so I went by that. Thanks for clarifying.</p>