Mathematical Statistics Lesson of the Day – An Example of An Ancillary Statistic

Consider 2 random variables, X_1 and X_2, from the normal distribution \text{Normal}(\mu, \sigma^2), where \mu is unknown.  Then the statistic

D = X_1 - X_2

has the distribution

\text{Normal}(0, 2\sigma^2).

The distribution of D does not depend on \mu, so D is an ancillary statistic for \mu.

Note that, if \sigma^2 is unknown, then D is not ancillary for \sigma^2.

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Mathematical Statistics Lesson of the Day – Complete Statistics

The set-up for today’s post mirrors my earlier Statistics Lesson of the Day on sufficient statistics.

Suppose that you collected data

\mathbf{X} = X_1, X_2, ..., X_n

in order to estimate a parameter \theta.  Let f_\theta(x) be the probability density function (PDF)* for X_1, X_2, ..., X_n.

Let

t = T(\mathbf{X})

be a statistic based on \mathbf{X}.

If

E_\theta \{g[T(\mathbf{X})]\} = 0, \ \ \forall \ \theta,

implies that

P \{g[T(\mathbf{X})]\} = 0] = 1,

then T(\mathbf{X}) is said to be complete.  To deconstruct this esoteric mathematical statement,

  1. let g(t) be a measurable function
  2. if you want to use g[T(\mathbf{X})] to form an unbiased estimator of the zero function,
  3. and if the only such function is almost surely equal to the zero function,
  4. then T(\mathbf{X}) is a complete statistic.

I will discuss the intuition behind this bizarre definition in a later Statistics Lesson of the Day.

*This above definition holds for discrete and continuous random variables.