This applet computes probabilities for the Poisson distribution:
$$X \sim Pois(\lambda)$$
It also can plot the likelihood, log-likelihood, asymptotic CI for $\lambda$, and determine the MLE and observed Fisher information.

#### Probability Mass Function (PMF) mode

Directions:

- Enter the rate in the $\lambda$ box.
- Hitting "Tab" or "Enter" on your keyboard will plot the probability mass function (pmf).

To compute a probability, select $P(X = x)$ from the drop-down box,
enter a numeric $x$ value, and press "Tab" or "Enter" on your keyboard. The probability
$P(X = x)$ will appear in the
pink box.
Select $P(X \le x)$ from the drop-down box for a left-tail probability (i.e. the cdf).

#### Maximum Likelihood (MLE) mode

Directions:

- Leave the $\lambda$ box empty.
- Enter the one or more data values in the $x$ box (separate multiple $x$ values by commas).
- Hitting "Tab" or "Enter" on your keyboard will plot the likelihood, log-likelihood, and 95% asymptotic CI for $\lambda$. The MLE and observed Fisher information are also displayed.

Note that the green line is drawn at $$max(log(likelihood))-\frac{3.84}{2}$$ The resulting asymptotic 95% CI is shown in green on the horizontal axis. The asymptotic CI is based on *large sample* theory; it is shown in this applet for all sample sizes, however.