Excess Kurtosis of Hat-Check Distribution
Jump to navigation
Jump to search
Theorem
Let $X$ be a discrete random variable with a Hat-Check distribution with parameter $n$. ($n \gt 3$)
Then the excess kurtosis $\gamma_2$ of $X$ is given by:
- $\gamma_2 = 1$
Proof
From Kurtosis in terms of Non-Central Moments, we have:
- $\gamma_2 = \dfrac {\expect {X^4} - 4 \mu \expect {X^3} + 6 \mu^2 \expect {X^2} - 3 \mu^4} {\sigma^4} - 3$
where:
- $\mu$ is the expectation of $X$.
- $\sigma$ is the standard deviation of $X$.
We have, by Expectation of Hat-Check Distribution:
- $\expect X = n - 1$
By Variance of Hat-Check Distribution:
- $\var X = \sigma^2 = 1$
- $\expect {X^2} = \paren {n - 1 }^2 + 1$
By Skewness of Hat-Check Distribution:
- $\expect {X^3} = n^3 - 3n^2 + 6n - 5$
To now calculate $\gamma_2$, we must calculate $\expect {X^4}$.
\(\ds \expect {X^4}\) | \(=\) | \(\ds \sum_{k \mathop = 0}^n {k^4 \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} }\) | Definition of Hat-Check Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{k \mathop = 1}^n {k^4 \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} }\) | as the $k = 0$ term vanishes | |||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{y \mathop = n - 1}^0 \paren {n - y }^4 \dfrac 1 {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!}\) | Let $y = n - k$ | |||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{y \mathop = 0}^{n - 1} \dfrac 1 {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{y \mathop = 0}^{n - 1} \dfrac y {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{y \mathop = 0}^{n - 1} \dfrac {y^2} {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!} - 4n\sum_{y \mathop = 0}^{n - 1} \dfrac {y^3} {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!} + \sum_{y \mathop = 0}^{n - 1} \dfrac {y^4} {y!} \sum_{s \mathop = 0}^{n - y} \dfrac {\paren {-1}^s} {s!}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 1}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 1}^n \dfrac {n - k} {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^2 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n\sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^3 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + \sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^4 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | Let $y = n - k$ | |||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 1}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 1}^n \dfrac {n - k} {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^2 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n\sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^3 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + \sum_{k \mathop = 1}^n \dfrac {\paren {n - k}^4 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 8 \dfrac {n^4} {n!} - 8 \dfrac {n^4} {n!}\) | adding $0$ - re-index sums | |||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 0}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 0}^n \dfrac {n - k} {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{k \mathop = 0}^n \dfrac {\paren {n - k}^2 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n\sum_{k \mathop = 0}^n \dfrac {\paren {n - k}^3 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + \sum_{k \mathop = 0}^n \dfrac {\paren {n - k}^4 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 0}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 0}^{n - 1} \dfrac {n - k} {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k}^2 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n\sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k}^3 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + \sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k}^4 } {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | as the $k = n$ term vanishes | |||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 0}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^2 \sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k} } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n\sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k}^2 } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + \sum_{k \mathop = 0}^{n - 1} \dfrac {\paren {n - k}^3 } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | canceling terms | |||||||||||
\(\ds \) | \(=\) | \(\ds n^4 \sum_{k \mathop = 0}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n^3 \sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 6n^3 \sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 6n^2 \sum_{k \mathop = 0}^{n - 1} \dfrac k {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | ||||||||||||
\(\ds \) | \(\) | \(\, \ds - \, \) | \(\ds 4n^3\sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 8n^2 \sum_{k \mathop = 0}^{n - 1} \dfrac k {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 4n \sum_{k \mathop = 0}^{n - 1} \dfrac {k^2 } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | |||||||||||
\(\ds \) | \(\) | \(\, \ds + \, \) | \(\ds n^3\sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - 3n^2 \sum_{k \mathop = 0}^{n - 1} \dfrac k {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} + 3n \sum_{k \mathop = 0}^{n - 1} \dfrac {k^2 } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!} - \sum_{k \mathop = 0}^{n - 1} \dfrac {k^3 } {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) |
To help complete the sum above, recall that:
\(\ds \sum_{k \mathop = 0}^n \dfrac 1 {\paren {n - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | \(=\) | \(\ds 1\) | Hat-Check Distribution Gives Rise to Probability Mass Function | |||||||||||
\(\ds \sum_{k \mathop = 0}^{n - 1} \dfrac 1 {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | \(=\) | \(\ds 1\) | Hat-Check Distribution Gives Rise to Probability Mass Function | |||||||||||
\(\ds \sum_{k \mathop = 0}^{n - 1} \dfrac k {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | \(=\) | \(\ds \paren {n - 1} - 1\) | Expectation of Hat-Check Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {n - 2}\) | ||||||||||||
\(\ds \sum_{k \mathop = 0}^{n - 1} \dfrac {k^2} {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | \(=\) | \(\ds \paren{\paren {n - 1} - 1}^2 + 1\) | Variance of Hat-Check Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren{n^2 -4n + 5}\) | ||||||||||||
\(\ds \sum_{k \mathop = 0}^{n - 1} \dfrac {k^3} {\paren {n - 1 - k }!} \sum_{s \mathop = 0}^k \dfrac {\paren {-1}^s} {s!}\) | \(=\) | \(\ds \paren{n - 1}^3 - 3\paren {n - 1}^2 + 6\paren {n - 1} - 5\) | Skewness of Hat-Check Distribution | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren{n^3 -6n^2 + 15n - 15}\) |
Therefore:
\(\ds \) | \(=\) | \(\ds n^4 - 4n^3 + 6n^3 - 6n^2 \paren {n - 2} - 4n^3 + 8n^2 \paren {n - 2} - 4n \paren {n^2 -4n + 5} + n^3 - 3n^2 \paren {n - 2} + 3n \paren {n^2 -4n + 5} - \paren {n^3 -6n^2 + 15n - 15}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds n^4 + \paren {-4 + 6 - 6 - 4 + 8 - 4 +1 - 3 + 3 - 1} n^3 + \paren {12 - 16 + 16 + 6 - 12 + 6 } n^2 + \paren {-20 + 15 - 15 } n + 15\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds n^4 - 4n^3 + 12n^2 - 20n + 15\) |
So:
\(\ds \gamma_2\) | \(=\) | \(\ds \frac {\paren {n^4 - 4n^3 + 12n^2 - 20n + 15 } - 4 \paren {n - 1} \paren {n^3 - 3n^2 + 6n - 5} + 6 \paren {n - 1}^2 \paren {n^2 - 2n + 2} - 3\paren {n - 1}^4} 1 - 3\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {n^4 - 4n^3 + 12n^2 - 20n + 15 } - \paren {4n^4 - 12n^3 + 24n^2 - 20n} + \paren {4n^3 - 12n^2 + 24n - 20}\) | Expand the second term | |||||||||||
\(\ds \) | \(\) | \(\, \ds + \, \) | \(\ds 6 \paren {n^2 - 2n + 1} \paren {n^2 - 2n + 2} - 3\paren {n^4 - 4n^3 + 6n^2 - 4n + 1} - 3\) | Expand powers: Binomial Theorem | ||||||||||
\(\ds \) | \(=\) | \(\ds n^4 - 4n^3 + 12n^2 - 20n + 15 - 4n^4 + 12n^3 - 24n^2 + 20n + 4n^3 - 12n^2 + 24n - 20\) | Remove parentheses | |||||||||||
\(\ds \) | \(\) | \(\, \ds + \, \) | \(\ds 6n^4 - 12n^3 + 12n^2 - 12n^3 + 24n^2 - 24n + 6n^2 - 12n + 12 - 3n^4 + 12n^3 - 18n^2 + 12n -3 - 3\) | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {1 - 4 + 6 - 3 }n^4 + \paren {-4 + 12 + 4 - 12 - 12 + 12 }n^3 + \paren {12 - 24 - 12 + 12 + 24 + 6 - 18 }n^2 + \paren {-20 + 20 + 24 - 24 - 12 + 12 }n + \paren {15 - 20 + 12 - 3 - 3 }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {0 }n^4 + \paren {0 }n^3 + \paren {0 }n^2 + \paren {0 }n + \paren {1 }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 1\) |
$\blacksquare$