Probability of secretary making 4 or more errors on a page I have this problem, and I want to figure out how to do it, or at least figure out the subject that it deals with. > A secretary who only does word processing makes $2$ errors per page when typing. What is the probability that in the next page she makes $4$ or more errors? Thank you!

You mean that the secretary makes an _average_ of two errors per page.

Usually, errors of this type are modelled using the **Poisson** distribution with parameter $\lambda$ equal to the mean number of errors per "unit," in this case page.

So if $X$ is the number of errors in a given page, our Poisson model gives $$\Pr(X=k)=e^{-2}\frac{2^k}{k!}.$$

The probability of $4$ or more errors is $1$ minus the probability of $3$ or fewer errors. And $$\Pr(X\le 3)=\sum_{k=0}^3 e^{-2}\frac{2^k}{k!}.$$

**Remark:** At best, the Poisson model will fit reality only modestly well.