# How can we use TransformedDistribution to infer the sampling distribution of the sample mean?

As we know, TransformedDistribution can infer a distribution of a transformation like

TransformedDistribution[A*X + B, X \[Distributed] NormalDistribution[μ, σ]]


NormalDistribution[A μ + B, σ Abs[A]]

But can we use it get the sampling distribution of the sample mean? Such as $$X_1,X_2,\cdots,X_n \sim \text{NormalDistribution}[μ, σ]$$ and then the $$\overline{X}=\frac{X_1,X_2,\cdots,X_n}{n}\sim \text{NormalDistribution}[μ, σ/\sqrt{n}]$$.

Can we do such symbolic derivation? My current try is

TransformedDistribution[Mean[{x, y, z}], Table[i \[Distributed] NormalDistribution[μ, σ], {i, {x, y, z}}]]


NormalDistribution[μ, Abs[σ]/Sqrt[3]]

I have to say it is not a real symbolic derivation. I hope to get a $$\text{NormalDistribution}[μ, σ/\sqrt{n}]$$ result. I am not sure I have missed anything.

PS: I mean, I don't want to get the number $$3$$ in the result. I hope to use $$n$$ parameters to get $$\text{NormalDistribution}[μ, σ/\sqrt{n}]$$. The result has that symbol $$n$$. But I am not sure MMA can do this. Like a real symbolic derivation.

Update: If you do not consider this cheating:

TransformedDistribution[w[n] / n, w \[Distributed] WienerProcess[μ, σ]]

NormalDistribution[μ, σ/Sqrt[n]]


Also

SliceDistribution[WienerProcess[μ/n, σ/n], n]

NormalDistribution[μ, σ/Sqrt[n]]


and

TransformedDistribution[x/n, x \[Distributed] SliceDistribution[WienerProcess[μ, σ], n]]

NormalDistribution[μ, σ/Sqrt[n]]


tdist[n_Integer] := TransformedDistribution[Mean[Array[x, n]],
Array[x, n] \[Distributed] ProductDistribution[{NormalDistribution[μ, σ], n}],
Assumptions -> {σ > 0}]

tdist /@ Range[4] // Column


n = 3;
mnd = MultinormalDistribution[Array[Subscript[μ, #] &, n],
Array[Subscript[σ, ##] &, {n, n}]];

TransformedDistribution[Mean[Array[Subscript[x, #] &, n]],
Array[Subscript[x, #] &, n] \[Distributed] mnd]


• I'm sorry, you just use 3 parameters to get $\text{NormalDistribution}[μ, σ/\sqrt{3}]$. But I hope to use $n$ parameters to get $\text{NormalDistribution}[μ, σ/\sqrt{n}]$
– yode
Jun 2 at 19:29
• @yode I don't know if it can be done with symbolic n.
– kglr
Jun 2 at 19:42
• +1 For whatever it's worth, I don't think it's cheating. Your update seems to me to be the answer.
– JimB
Jun 3 at 3:30
• Wow, awesome!!, thanks very very much. Can we use your update method to calculate the The Sampling Distribution of the sample sum-of-squares? In theory, the result is $χ^2$.
– yode
Jun 3 at 3:58
• Stealing from your answer using TransformedDistribution[Sum[w[i]^2, {i, 1, n}], w \[Distributed] WhiteNoiseProcess[NormalDistribution[0, \[Sigma]]]] gives the correct distribution when n is defined but gives nothing recognizable for the distribution and the variance is wrong when n isn't defined.
– JimB
Jun 3 at 18:12

This answer also does not use TransformedDistribution but does come up with the correct variance and that the distribution is normal using an unspecified sample size. (My other answer only deals with getting the correct variance.) But I'm assuming that if TransformedDistribution can't be made to work, then you want some method that works for an unspecified sample size.

This solution uses moment generating functions. (One could also use characteristic functions.)

The moment generating function of a normal distribution with mean $$\mu$$ and variance $$\sigma^2$$ is

mgf = MomentGeneratingFunction[NormalDistribution[μ, σ], t]


$$e^{\frac{\sigma ^2 t^2}{2}+\mu t}$$

The moment generating function of a sum of normals is found by raising the mgf to the power $$n$$:

mgfSum = mgf^n /. Exp[a_]^n -> Exp[a n] // ExpandAll


$$e^{\frac{1}{2} n \sigma ^2 t^2+\mu n t}$$

Finally get the mgf of the mean which is the sum divided by $$n$$:

mgfxbar = mgfSum /. t -> t/n // ExpandAll


$$e^{\frac{\sigma ^2 t^2}{2 n}+\mu t}$$

Doing all of this in a single command is

MomentGeneratingFunction[NormalDistribution[μ, σ], t/n]^n /. Exp[a_]^n -> Exp[a n] // ExpandAll


$$e^{\frac{\sigma ^2 t^2}{2 n}+\mu t}$$

So the sample mean also has a normal distribution with mean $$\mu$$ and variance $$\sigma^2/n$$ because a moment generating function uniquely identifies the distribution.

The following might be an approach to get moments using an unspecified sample size. (Admittedly, it's bit circular/odd/obtuse. I'm sure it can be explained better.)

The sample mean is

xbar = Sum[x[i], {i, 1, n}]/n


This can be represented using the PowerSymmetricPolynomial function:

xbar = PowerSymmetricPolynomial[1]/n


where PowerSymmetricPolynomial[k] represents

Sum[x[i]^k, {i, 1, n}]


If you're willing to allow the expectation of PowerSymmetricPolynomial[k] for $$k=0,1,2$$ to be n, n Moment[1], and n Moment[2], respectively, then we write some expectation rules:

expectationRules = {PowerSymmetricPolynomial[0] -> n,
PowerSymmetricPolynomial[1] -> n Moment[1],
PowerSymmetricPolynomial[2] -> n Moment[2]}


We also write some notational rules to put things in customary form:

notationRules = {Moment[1] -> μ, Moment[2] -> σ^2 + μ^2, CentralMoment[2] -> σ^2}


Putting this altogether using the function MomentConvert:

Eofxbar = xbar /. expectationRules /. notationRules
(* μ *)

varxbar = MomentConvert[(xbar - Eofxbar)^2, "CentralMoment"] /. expectationRules /. notationRules
(* σ^2/n *)

• I recognize I seem to be on a kick of "not answering the direct question." But It seems the real question is how to get results with an unspecified sample size whether or not TransformedDistribution will do the job directly.
– JimB
Jun 2 at 21:16
Clear["Global*"]

$Assumptions = Element[m, Reals] && s > 0;  The distribution of the Mean of n i.i.d. normal random variables is dist[n_Integer?Positive] := dist[n] = TransformedDistribution[ Sum[x[k], {k, 1, n}]/n, Table[x[k] \[Distributed] NormalDistribution[m, s], {k, 1, n}]]  The distributions are all normal Union[Head /@ (dist /@ Range[5])] (* {NormalDistribution} *)  The Mean is constant μ[n_] = FindSequenceFunction[Mean[dist[#]] & /@ Range[5], n] (* m *)  The Variance is var[n_] = FindSequenceFunction[Variance[dist[#]] & /@ Range[5], n] (* s^2/n *)  σ[n_] = Sqrt[var[n]] // Simplify (* Sqrt[1/n] s *)  The general result is then NormalDistribution[μ[n], σ[n]] (* NormalDistribution[m, Sqrt[1/n] s] *)  Checking, And @@ Table[dist[n] == NormalDistribution[μ[n], σ[n]], {n, 1, 20}] (* True *)  If I correctly understand the question, it can be done as follows. Table[TransformedDistribution[Total[Table[x[i], {i, 1, n}]]/n, Table[x[i] \[Distributed] NormalDistribution[\[Mu], \[Sigma]], {i, 1, n}]], {n, 1, 4}]  {NormalDistribution[\[Mu], \[Sigma]], NormalDistribution[\[Mu], Abs[\[Sigma]]/Sqrt[2]], NormalDistribution[\[Mu], Abs[\[Sigma]]/Sqrt[3]], NormalDistribution[\[Mu], Abs[\[Sigma]]/2]} Addition. How about that tricky way (TransformedDistribution deals only with concrete sums.)? n = 3; z = TransformedDistribution[Total[Table[x[i], {i, 1, n}]]/n, Table[x[i] \[Distributed] NormalDistribution[\[Mu], \[Sigma]], {i, 1, n}]]; Clear[n]; Replace[z, 3 -> n, 3]  NormalDistribution[\[Mu], Abs[\[Sigma]]/Sqrt[n]] • Yes, I have understood it correctly. If my English mislead the topic. Could you help me fix it? – yode Jun 2 at 19:09 • As your answer, it seems I have misused the 2nd parameter in the TransformedDistribution... – yode Jun 2 at 19:17 • As your answer, it seems I have misused the 2nd parameter in the TransformedDistribution. So I have adjusted the question to show my question exactly. – yode Jun 2 at 19:28 • Hey man, :) I feel crazy to see your newest modify. I don't know why my English so hard to express the real meaning.. You just knock together a$n\$ in the result. :)
– yode
Jun 2 at 19:39
• @yode: I did my best. Deep regard. Jun 2 at 19:42

This is doesn't work, but I've not deleted it as I think it's interesting

You can do a lot of this purely symbolically. For example, you can define a transformed distribution giving the sum of an arbitrary number of IID random variables (with an arbitrary distribution).

\[ScriptD] =
TransformedDistribution[1/n Sum[Indexed[x, i], {i, 1, n}],
Indexed[x, i] \[Distributed] \[ScriptCapitalD]];


You can apply Mean and Variance to this...

{Mean[\[ScriptD]], Variance[\[ScriptD]]};


... but it doesn't do anything useful, until you substitute a specific distribution...

{Mean[\[ScriptD]],
Variance[\[ScriptD]]} /. {{\[ScriptCapitalD] ->
NormalDistribution[μ, σ]}, {\[ScriptCapitalD] ->
ExponentialDistribution[1/μ]}}
(* {{μ, σ^2}, {μ, μ^2}} *)


And as you wished, substituting specific distributions give the distributions of the sample mean

\[ScriptD] /. {{\[ScriptCapitalD] ->
NormalDistribution[μ, σ]}, {\[ScriptCapitalD] ->
ExponentialDistribution[1/μ]}} // FullSimplify
(* {NormalDistribution[μ, σ],
ExponentialDistribution[1/μ]} *)


Unfortunately, as you'll note the answers are wrong. It assumes that all the Indexed[x,i]` are the same value