Poisson distribution
Counts the number of events during a fixed interval of length T.
P(N(T) = k) = e−λT(λT)k / k!
Variable type: discrete.
Possible values: 0, 1, 2, 3, …
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Counts the number of events during a fixed interval of length T.
Variable type: discrete.
Possible values: 0, 1, 2, 3, …
Measures the continuous waiting time between two consecutive events.
Variable type: continuous.
Possible values: every non-negative real number.
Each vertical block represents one fixed interval of length T.
Each event creates a new interarrival-time observation.
| Characteristic | Poisson | Exponential |
|---|---|---|
| Question answered | How many events occur during a fixed interval? | How long must we wait until the next event? |
| Random variable | Number of events | Time between events |
| Variable type | Discrete | Continuous |
| Expected value | E[N(T)] = λT | E[X] = 1/λ |
| Typical logistics example | Number of trucks arriving at a gate within one hour. | Time between consecutive truck arrivals. |