## STAT430 : Questions

Referers: Fall2007 :: (Remote :: Orphans :: Tree )
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This is an old revision of Questions from 2007-10-10 10:19:41.
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• I have been reviewing the special pmf's and am a little confused by E[X] for Geometric (packet 1, p.29).  $p X ( x )$ makes sense, as do the results of the expectancy and variance, but the sum gets me.  If we went over this I apologize, but what does the leading $i$ represent?  If we expand is it to show that in order to get to $i$ we must have the $i -1 , i -2 , ... , i - z +1$ failures before the $z$th trial succeeds?

Yes, there is a typo in the formula for $E [ X ]$.  Here is corrected formula plus detailed derivation.

• There appears to be a typo in the derivation of E[X] for the Poisson distribution (packet 1, p.31).  The x that is factored out and canceled with the leading x in x! to make the denominator (x-1)! reappears in the next line.  It shouldn't.

Agreed.  My notes for derivation of $E [ X ]$ read simpler.

• We know that if we have a Poisson distribution E[X] = Var[X].  Is this an iff statement?  That is, if we have that E[X] = Var[X] are we guaranteed that our random variable is described by the Poisson distribution?

It is not an iff statement.  One must have all moments  match (when they exist) with those of a known distribution to conclude that a random variable has this distribution.  See moment generating function .  Thus, we would also have to check higher moments, like $E [ X 3 ]$, match those of a Poisson random variable to conclude that $X ∼$ Poisson.

• I am having difficulty with the definition of $Ω$.  Initially we defined $Ω$ as the set of all possible outcomes (for probability).  However, when we defined a random variable we said that a random variable $X$ was defined as a mapping from $Ω$ onto $R$.  That, to me, says that $Ω$ is the domain and $R$ the codomain.  These seem contradictory.  Is this because one is a definition for probabilities and the other for statistics, or is there something I am misinterpreting?   This question was prompted because I have in my notes that $Ω X$ is the range of $X$.  Assuming that to be true I was then thinking that a transformation of two random variables $X$,$Y$ is like a composition function; it is a mapping from $R$, through $Ω X$, to $Ω Y$ that is surjective but not injective.  The correctness of this is obviously dependent on the definition of these $Ω$ sets...

Yes, this is abusive notation.  So, $Ω$ is the sample space consisting of all possible outcomes of a random experiment.  A random variable maps $Ω$ to some subspace of $R$.  If we sort of forget about the random experiment and outcomes, and treat the random variable as the outcome, then we can call this $R$ subspace $Ω X$.  Proper, careful notation would probably use something other than $Ω$ for this purpose.

• In the chi-square test of independence, why is the degrees of freedom $( n r - 1 ) ( n c -1 )$ where $n r$ is the number of rows and $n c$ is the number of columns.

As per our discussion about goodness-of-fit tests, the degrees of freedom should be $m -1$ less the number of parameters estimated, where $m$ is the number of categories.  In the test of independence, the number of categories is $n r n c$.  Under independence, there are $n r -1$ parameters to estimate for the marginal pmf on rows, one for each category minus the constraint that the pmf $∑ i p i = 1$ sums to one.  Similarly, there are $n c -1$ additional parameters to estimate for the pmf on columns.  Therefore, the number of degrees of freedom is $n r n c - ( n r -1 ) - ( n c -1 ) -1 = ( n r -1 ) ( n c -1 )$, in agreement with the rule for tests of independence.  In conclusion, the test of independence can be viewed as a special type of goodness-of-fit test.

• Is there any course schedule that we can know what we will learn during this semester? That would helpful for deciding which group project is suitable. Thanks.
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