Posts Tagged ‘moment generating function’

Probability manipulations with Mathematica…

There comes a time that a statistician needs to do some ananytic calculations. There more than a bunch of tools to use but I usually prefer Mathematica or Maple. Today, I’m gonna use Mathematica to do a simple exhibition.

Let’s set this example upon the  U(2 \theta _1-\theta _2\leq x\leq 2 \theta _1+\theta _2)   distribution.

pfun = PDF[UniformDistribution[{2*Subscript[θ, 1] - Subscript[θ, 2],
2*Subscript[θ, 1] + Subscript[θ, 2]}], x]

\begin{cases}  \frac{1}{2 \theta _2} & 2 \theta _1-\theta _2\leq x\leq 2 \theta _1+\theta _2 \\  0 & \text{True}  \end{cases}

One of the most intensive calculations is the characteristic function (eq. the moment generating function). This is straightforward to derive.

{2*Subscript[θ, 1]-Subscript[θ, 2],2*Subscript[θ, 1]+Subscript[θ, 2]}],x]

-\frac{i \left(-e^{i x \left(2 \theta _1-\theta _2\right)}+e^{i x \left(2 \theta _1+\theta _2\right)}\right)}{2 x \theta _2} .

The Table[] command calculates for us the raw moments for our distribution.

Table[Limit[D[cfun, {x, n}], x -> 0]/I^n, {n, 4}]

\left\{2 \theta _1,\frac{1}{3} \left(12 \theta _1^2+\theta _2^2\right),2 \theta _1 \left(4 \theta _1^2+\theta _2^2\right),16 \theta _1^4+8 \theta _1^2 \theta _2^2+\frac{\theta _2^4}{5}\right\} .

Calculate the sample statistics.


\{4.99333,8.46171\} .

Now, we can use a simple moment matching technique to get estimates for the parameters.

Solve[{Mean[T]-2*Subscript[θ, 1]==0,-(2*Subscript[θ, 1])^2+
1/3 (12 Subscript[θ, 1]^2+\!\*SubsuperscriptBox[\(θ\), \(2\), \(2\)])-
Variance[T]==0},{Subscript[θ, 2],Subscript[θ, 1]}]

\left\{\left\{\theta _1\to 2.49667,\theta _2\to -5.03836\right\},\left\{\theta _1\to 2.49667,\theta _2\to 5.03836\right\}\right\} .

Check the true value for the \theta _2.

Reduce[2 Subscript[θ, 1]-Subscript[θ, 2]<=2 Subscript[θ, 1]+Subscript[θ, 2],
Subscript[θ, 2]]

\theta _1\in \text{Reals}\&\&\theta _2\geq 0 .

Then, \left\{\left\{\theta _1\to 2.49667,\theta _2\to  5.03836\right\}\right\} .