WEBVTT
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Hey, everyone.
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Welcome back.
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Today, we're going to do another
fun problem that has
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to do with a random number
of coin flips.
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So the experiment we're going
to run is as follows.
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We're given a fair six-sided
die, and we roll it.
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And then we take a fair coin,
and we flip it the number of
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times indicated by the die.
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That is to say, if I roll a four
on my die, then I flip
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the coin four times.
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And then we're interested in
some statistics regarding the
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number of heads that show
up in our sequence.
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In particular, we want to
compute the expectation and
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the variance of the number
of heads that we see.
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So the first step of this
problem is to translate the
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English to the math.
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So we have to define
some notation.
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I went ahead and did
that for us.
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I defined n to be the outcome
of the die role.
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Now, since we flip the coin the
number of times shown by
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the die roll, n is equivalently
the number of
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flips that we perform.
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And n, of course, is a random
variable, and I've written its
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PMF up here.
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So Pn of n is just a discrete
uniform random variable
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between 1 and 6, because we're
told that the die has six
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sides and that it's fair.
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Now, I also defined h
to be the number of
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heads that we see.
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So that's the quantity
of interest.
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And it turns out that Bernoulli
random variables
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will be very helpful to
us in this problem.
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So I defined x sub i as
1 if the ith flip is
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heads, and 0 otherwise.
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And what we're going to do now
is, we're going to use these x
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sub i's to come up with
an expression for h.
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So if you want to count the
number of heads, one possible
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thing you could do is start with
0 and then look at the
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first coin flip.
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If it's heads, you add 1 to 0,
which I'm going to call your
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running sum.
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If the first flip is
tails, you add 0.
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And similarly, after that, after
every trial, if you see
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heads, you add 1 to
your running sum.
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If you see a tails, you add 0.
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And in that way, we can
precisely compute h.
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So the mathematical statement of
what I just said is that h
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is equal to x1 plus x2
plus x3, all the way
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through x sub n.
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So now, we are interested
in computing e of h, the
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expectation of h.
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So your knee jerk reaction might
be to say, oh, well, by
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linearity of expectation,
we know that this is an
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expectation of x1, et cetera
through the expectation of xn.
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But in this case, you would
actually be wrong.
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Don't do that.
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And the reason that this is not
going to work for us is
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because we're dealing
with a random
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number of random variables.
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So each xi is a random
variable.
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And we have capital n of them.
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But capital n is a
random variable.
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It denotes the outcome
of our die roll.
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So we actually cannot just
take the sum of these
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expectations.
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Instead, we're going to have
to condition on n and use
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iterated expectation.
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So this is the mathematical
statement of what I just said.
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And the reason why this works
is because conditioning on n
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will take us to the case that
we already know how to deal
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with, where we have a known
number of random variables.
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And of course, iterated
expectations holds, as you saw
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in lecture.
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I will briefly mention here that
the formula we're going
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to derive is derived
in the book.
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And it was probably derived
in lecture.
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So if you want, you can just
go to that formula
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immediately.
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But I think the derivation of
the formula that we need is
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quick and is helpful.
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So I'm going to go through
it quickly.
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Let's do it over here.
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Plugging in our running
sum for h, we get this
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expression--
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x1 plus x2 et cetera plus
xn, conditioned on n.
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And this, of course,
is n times the
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expectation of x sub i.
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So again, I'm going through
this quickly,
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because it's in the book.
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But this step holds, because
each of these xi's have the
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same statistics.
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They're all Bernoulli with
parameter of 1/2, because our
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coin is fair.
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And so I used x sub i to say it
doesn't really matter which
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integer you pick for i, because
the expectation of xi
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is the same for all i.
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So this now, the expectation
of x sub i, this is just a
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number, it's just some constant,
so you can pull it
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out of the expectation.
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So you get the expectation
of x sub i times the
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expectation of n.
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So I gave away the answer
to this a second ago.
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But x sub i is just a Bernoulli
random variable with
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parameter of success of 1/2.
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And we know already that the
expectation of such a random
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variable is just p, or 1/2.
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So this is 1/2 times
expectation of n.
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And now n we know is a discrete
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uniform random variable.
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And there's a formula that I'm
going to use, which hopefully
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some of you may remember.
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If you have a discrete uniform
random variable that takes on
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values between a and b--
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let's use w--
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if you call this random variable
w, then we have that
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the variance of w is equal to b
minus a times b minus a plus
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2 divided by 12.
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So that's the variance.
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We don't actually need the
variance, but we will need
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this later.
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And the expectation of w--
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actually, let's just
do it up here right
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ahead for this problem.
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Because we have a discrete
uniform random variable, the
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expectation is just
the middle.
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So you agree hopefully that the
middle is right at 3.5,
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which is also 7/2.
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So this is times 7/2, which
is equal to 7/4.
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So we are done with
part of part a.
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I'm going to write this answer
over here, so I can erase.
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And we're going to do something
very similar to
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compute the variance.
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To compute the variance,
we are going to also
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condition on n.
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So we get rid of this source
of randomness.
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And then we're going to use law
of total variance, which
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you've also seen in lecture.
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And again, the formula
for this variance is
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derived in the book.
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So I'm going to go through
it quickly.
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But make sure you understand
this derivation, because it
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exercises a lot of stuff
we taught you.
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So this, just using law of
total variance, is the
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variance of expectation of h
given n, plus the expectation
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of the variance of h given n.
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And now, plugging in
this running sum
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for h, you get this.
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It's a mouthful to write.
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Bear with me.
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x1 through xn given n--
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so I didn't do anything fancy.
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I just plugged this into here.
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So this term is similar
to what we saw
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in a previous problem.
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By linearity of expectation and
due to the fact that all
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of the x i's are distributed in
the same way, they have the
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same expectation, this
becomes n times the
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expectation of x sub i.
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And let's do this
term over here.
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This term--
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well, conditioned on
n, this n is known.
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So we essentially have
a finite known sum of
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independent random variables.
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We know that the variance of
a sum of independent random
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variables is the sum
of the variances.
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So this is the variance of x1
plus the variance of x2 et
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cetera, plus the
variance of xn.
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And furthermore, again, because
all of these xi's have
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the same distribution, the
variance is the same.
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So we can actually write this
as n times the variance of x
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sub i, where x sub i
just corresponds
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to one of the trials.
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It doesn't matter which one,
because they all have the same
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variance and expectation.
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So now, we're almost
home free.
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This is just some scaler.
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So we can take it out of
the variance, but we
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have to square it.
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So this becomes expectation
of xi squared times
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the variance of n.
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And then this variance is also
just a scalar, so we can take
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it outside.
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So then we get variance of x sub
i times expectation of n.
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Now, we know that the
expectation of x sub i is just
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the probability of success,
which is 1/2.
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So we have 1/2 squared, or 1/4,
times the variance of n.
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So that's where this formula
comes in handy.
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b is equal to 6, a
is equal to 1.
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So we get that the variance
of n is equal to 5 times--
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and then 5 plus 2 is 7--
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divided by 12.
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So this is just a formula from
the book that you guys
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hopefully remember.
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So we get 35/12.
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And then the variance of xi,
we know the variance of a
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Bernoulli random variable is
just p times 1 minus p.
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So in our case, that's 1/2
times 1/2, which is 1/4.
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So we get 1/4.
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And then the expectation of n,
we remember from our previous
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computation, is just 7/2.
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So I will let you guys do this
arithmetic on your own time.
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But the answer comes
out to be 77/48.
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So I will go ahead and put
our answer over here--
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77/48--
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so that I can erase.
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So I want you guys to
start thinking about
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part b while I erase.
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Essentially, you do the same
experiment that we did in part
00:10:49.330 --> 00:10:54.840
a, except now we use two
dice instead of one.
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So in part b, just to repeat,
you now have two dice.
00:10:58.690 --> 00:11:00.510
You roll them.
00:11:00.510 --> 00:11:01.600
You look at the outcome.
00:11:01.600 --> 00:11:05.220
If you have an outcome of four
on one die and six on another
00:11:05.220 --> 00:11:08.040
die, then you flip the
coin 10 times.
00:11:08.040 --> 00:11:09.880
So it's the same exact
experiment.
00:11:09.880 --> 00:11:11.920
We're interested in the number
of heads we want the
00:11:11.920 --> 00:11:14.400
expectation and the variance.
00:11:14.400 --> 00:11:17.040
But this step is now a
little bit different.
00:11:20.380 --> 00:11:24.560
Again, let's approach this by
defining some notation first.
00:11:24.560 --> 00:11:30.780
Now, I want to let n1 be the
outcome of the first die.
00:11:35.200 --> 00:11:39.450
And then you can let n2 be the
outcome of the second die.
00:11:45.010 --> 00:11:46.900
And we'll start with
just that.
00:11:46.900 --> 00:11:51.240
So one way you could approach
this problem is say, OK, if n1
00:11:51.240 --> 00:11:54.690
is the outcome of my first die
and n2 is the outcome of my
00:11:54.690 --> 00:11:57.720
second die, then the number of
coin flips that I'm going to
00:11:57.720 --> 00:11:59.250
make is n1 plus n2.
00:12:03.460 --> 00:12:05.650
This is the total coin flips.
00:12:08.570 --> 00:12:12.830
So you could just repeat the
same exact math that we did in
00:12:12.830 --> 00:12:17.610
part a, except everywhere that
you see an n, you replace that
00:12:17.610 --> 00:12:20.850
n with n1 plus n2.
00:12:20.850 --> 00:12:23.730
So that will get you to your
answer, but it will require
00:12:23.730 --> 00:12:25.400
slightly more work.
00:12:25.400 --> 00:12:27.590
We're going to think about
this problem slightly
00:12:27.590 --> 00:12:29.510
differently.
00:12:29.510 --> 00:12:33.430
So the way we are thinking about
it just now, we roll two
00:12:33.430 --> 00:12:35.040
dice at the same time.
00:12:35.040 --> 00:12:38.630
We add the results
of the die rolls.
00:12:38.630 --> 00:12:43.560
And then we flip the coin
that number of times.
00:12:43.560 --> 00:12:48.540
But another way you can think
about this is, you roll one
00:12:48.540 --> 00:12:51.380
die, and then you flip the coin
the number of times shown
00:12:51.380 --> 00:12:53.990
by that die and count
the number of heads.
00:12:53.990 --> 00:12:57.410
And then you take the second
die and you roll it.
00:12:57.410 --> 00:13:02.960
And then you flip the coin that
many more times and count
00:13:02.960 --> 00:13:05.600
the number of heads
after that.
00:13:05.600 --> 00:13:16.140
So you could define h1 to be
number of heads in the first
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n1 coin flips.
00:13:21.670 --> 00:13:29.940
And you could just let h2 be
the number of heads in the
00:13:29.940 --> 00:13:34.620
last n2 coin flips.
00:13:34.620 --> 00:13:38.320
So hopefully that terminology
is not confusing you.
00:13:38.320 --> 00:13:42.990
Essentially, what I'm saying
is, n1 plus n2 means you'll
00:13:42.990 --> 00:13:51.082
have n1 flips, followed by
n2 flips, for a total
00:13:51.082 --> 00:13:53.330
of n1 plus n2 flips.
00:13:53.330 --> 00:13:56.300
And then within the first n1
flips, you can get some number
00:13:56.300 --> 00:13:59.390
of heads, which we're
calling h1.
00:13:59.390 --> 00:14:02.200
And in the last n2 flips, you
can get some number of heads,
00:14:02.200 --> 00:14:04.340
which is h2.
00:14:04.340 --> 00:14:08.400
So the total number of heads
that we get at the end--
00:14:08.400 --> 00:14:10.351
I'm going to call it h star--
00:14:10.351 --> 00:14:14.590
is equal to h1 plus h2.
00:14:14.590 --> 00:14:17.370
And what part b is really
asking us for is the
00:14:17.370 --> 00:14:22.200
expectation of h star and
the variance of h star.
00:14:22.200 --> 00:14:26.406
But here's where something
really beautiful happens.
00:14:26.406 --> 00:14:31.220
h1 and h2 are independent,
and they are
00:14:31.220 --> 00:14:33.000
statistically the same.
00:14:33.000 --> 00:14:36.760
So the reason why they're
independent is because--
00:14:36.760 --> 00:14:40.200
well, first of all, all of our
coin flips are independent.
00:14:40.200 --> 00:14:44.560
And they're statistically the
same, because the experiment
00:14:44.560 --> 00:14:45.650
is exactly the same.
00:14:45.650 --> 00:14:47.110
And everything's independent.
00:14:47.110 --> 00:14:52.940
So instead of imagining one
person rolling two die and
00:14:52.940 --> 00:14:55.390
then summing the outcomes and
flipping a coin that many
00:14:55.390 --> 00:14:58.670
times and counting heads, you
can imagine one person takes
00:14:58.670 --> 00:15:00.720
one die and goes
into one room.
00:15:00.720 --> 00:15:02.710
A second person takes a
second die and goes
00:15:02.710 --> 00:15:04.500
into another room.
00:15:04.500 --> 00:15:06.100
They run their experiments.
00:15:06.100 --> 00:15:08.520
Then they report back
to a third person
00:15:08.520 --> 00:15:09.930
the number of heads.
00:15:09.930 --> 00:15:13.460
And that person adds them
together to get h star.
00:15:13.460 --> 00:15:16.660
And in that scenario, everything
is very clearly
00:15:16.660 --> 00:15:18.560
independent.
00:15:18.560 --> 00:15:21.330
So the expectation of h star--
00:15:21.330 --> 00:15:23.560
you actually don't need
independence for this part,
00:15:23.560 --> 00:15:26.580
because linearly of expectation
always holds.
00:15:26.580 --> 00:15:31.450
But you get the expectation of
h1 plus the expectation of h2.
00:15:31.450 --> 00:15:35.200
And because these guys are
statistically equivalent, this
00:15:35.200 --> 00:15:39.700
is just two times the
expectation of h.
00:15:39.700 --> 00:15:42.840
And the expectation of h we
calculated in part a.
00:15:42.840 --> 00:15:47.760
So this is 2 times 7 over 4.
00:15:47.760 --> 00:15:49.670
Now, for the variance,
here's where the
00:15:49.670 --> 00:15:50.930
independence comes in.
00:15:50.930 --> 00:15:54.238
I'm actually going to write this
somewhere where I don't
00:15:54.238 --> 00:15:55.200
have to bend over.
00:15:55.200 --> 00:16:00.690
So the variance of h star is
equal to the variance of h1
00:16:00.690 --> 00:16:03.820
plus the variance of
h2 by independence.
00:16:03.820 --> 00:16:07.020
And that's equal to 2 times
the variance of h, because
00:16:07.020 --> 00:16:09.740
they are statistically
the same.
00:16:09.740 --> 00:16:12.810
And the variance of h
we computed already.
00:16:12.810 --> 00:16:18.650
So this is just 2 times
77 over 48.
00:16:18.650 --> 00:16:23.390
So the succient answer to part
b is that both the mean and
00:16:23.390 --> 00:16:28.830
the variance double from part
A. So hopefully you guys
00:16:28.830 --> 00:16:30.060
enjoyed this problem.
00:16:30.060 --> 00:16:32.510
We covered a bunch of things.
00:16:32.510 --> 00:16:37.870
So we saw how to deal with
having a random number of
00:16:37.870 --> 00:16:38.830
random variables.
00:16:38.830 --> 00:16:41.840
Usually we have a fixed number
of random variables.
00:16:41.840 --> 00:16:44.670
In this problem, the number of
random variables we were
00:16:44.670 --> 00:16:47.350
adding together was
itself random.
00:16:47.350 --> 00:16:49.410
So to handle that, we
conditioned on n.
00:16:49.410 --> 00:16:53.110
And to compute expectation, we
use iterated expectation.
00:16:53.110 --> 00:16:57.450
To compute variance, we used
law of total variance.
00:16:57.450 --> 00:17:03.020
And then in part b, we were
just a little bit clever.
00:17:03.020 --> 00:17:07.270
We thought about how can we
reinterpret this experiment to
00:17:07.270 --> 00:17:08.859
reduce computation.
00:17:08.859 --> 00:17:12.869
And we realized that part b is
essentially two independent
00:17:12.869 --> 00:17:14.150
trials of part a.
00:17:14.150 --> 00:17:15.900
So both the mean and the
variance should double.