# Estimator for $E[X]^2$

I'm trying to understand the theory of estimators. As I understand it now, if you have an r.v. $X$ and take $n$ i.i.d. samples then an estimator for $E[X^{2}]$ would be $\overline{X^{2}}$ since $E[\overline{X^{2}}] = E[X^{2}]$ (probably only true for some kind of "nice" r.v.).

However, the same kind of nice result doesn't occur when trying to estimate $E[X]^{2}$. That is to say, the function $\overline{X}^{2}$ does not estimate this. But I'm not sure I understand which function does estimate it. I have the equation $V[\overline{X}] = E[\overline{X}^{2}]-E[\overline{X}]^{2}$ and so $E[X]^{2} = E[\overline{X}^{2}]-V[\overline{X}]$. This seems relevant but I'm not sure what to conclude from this.

• Actually the natural estimator for $E \left[ X^2 \right]$ is $\frac{1}{n} \sum_{i=1}^n X_i ^2$. Can you show that it is unbiased for the case of iid samples? – JohnK Oct 22 '14 at 14:53
• @JohnK I believe that what the OP means by "$\overline{X^{2}}$" is precisely $\frac{1}{n} \sum_{i=1}^n X_i ^2$. – whuber Oct 22 '14 at 15:02
• Addem, any statistic will estimate $E[X]^2$. The right question to ask is how well will it do. The answer depends on how you measure the goodness of an estimator. – whuber Oct 22 '14 at 15:04
• @whuber thats a great point, good way to step back and look at the bigger picture. – bdeonovic Oct 22 '14 at 16:56

By continuous mapping theorem $\bar{X}^2 \to \text{E}[X]^2$ in probability, so I would say it is a good estimate.

Depending on the distribution of $X$, if $\bar{X}$ is the MLE of $\text{E}[X]$, then $\bar{X}^2$ will be the MLE of $\text{E}[X]^2$ (MLE is invariant to transformation).

If $X_i$ are iid and $\text{Var}[X] = \sigma^2$, then $\text{Var}[\bar{X}] = \text{Var}\left[ \dfrac{1}{n} \sum_{i=1}^n X_i\right] = \dfrac{1}{n^2}\sum_{i=1}^n \text{Var}[X_i] = \dfrac{\sigma^2}{n}$

Background: unbiased estimators of products of population moments

If you desire an UNBIASED estimator of a (product of moments), there are 3 varieties:

1. Polykays (a generalisation of k-statistics): these are unbiased estimators of products of population cumulants. The term polykay was coined by Tukey, but the concept goes back to Dressel (1940).

2. Polyaches (a generalisation of h-statistics): these are unbiased estimators of products of population central moments. i.e.

$$E\left[\text{h}_{\{r,t,\ldots ,v\}}\right] = {\mu }_r {\mu }_t \cdots {\mu }_v\text{\, }$$ ...... where ${\mu }_r$ denotes the $r^{th}$ central moment of the population.

1. Polyraws: these are unbiased estimators of products of population raw moments. That is, you wish to find the $polyraw_{r, t, ...v}$ such that:

$$E\left[\text{polyraw}_{\{r,t,\ldots ,v\}}\right] = \acute{\mu }_r \acute{\mu }_t \cdots \acute{\mu }_v\text{\, }$$

...... where $\acute{\mu }_r$ denotes the $r^{th}$ raw moment of the population.

The Problem

We are given a random sample $(X_1, X_2, \dots, X_n)$ drawn on parent random variable $X$.

If we desire an unbiased estimator of: $(E[X])^2 = \acute{\mu }_1 \acute{\mu }_1$, then an unbiased estimator is the {1,1} polyraw: where $s_r = \sum_{i=1}^n X_i^r$ denotes the $r^{th}$ power sum.

Comparison

Benjamin proposed the estimator: $\bar{X}^2 = (\frac{s_1}{n})^2$. This is not an unbiased estimator, since $E[(\frac{s_1}{n})^2]$ is just the $1^{st}$ RawMoment of $(\frac{s_1}{n})^2$: which is not equal to $\acute{\mu }_1^2$.

Let us check the polyraw solution: ... which is an unbiased estimator.

Plainly, unbiasedness is not everything, and we could equally calculate, for example, the MSE (mean-squared error) of each estimator using exactly the same tools.

[Update: Just had a quick play with this: in a simple test case of $X \sim N(0,\sigma^2)$, the polyraw unbiased estimator has smaller MSE than Ben's ML estimator, for all sample sizes $n$. That is, at least for the test case of Normality, the polyraw unbiased estimator dominates the maximum likelihood estimator, at all sample sizes. ]

Notes

• PolyRaw, RawMomentToRaw etc are functions in the mathStatica package for Mathematica

• I confess to the neologism polyache in our Springer book (2002) (and more recently, to polyraw in the latest edition).

• Took me a while to guess you meant to pronounce that "polyache" as 'poly-aitch' (poly-"h"). – Glen_b -Reinstate Monica Oct 22 '14 at 20:33
• The lower MSE is very interesting (because simple situations where MLE loses to moments even in fairly small samples don't come along all that often). This question seeks examples where method of moments beats MLE in such a manner. – Glen_b -Reinstate Monica Oct 22 '14 at 21:29