Expectation by HC111124182648

VIEWS: 15 PAGES: 20

									                             Exercises on Expectations

Let X be a random variable with the distribution:       x        Pr(X=x)
                                                        1         20%
What is E[X] ?                           2.75           2         15%      #NAME?
                                                        3         35%
                                                        4         30%

What is E[X2] ?

The probability distribution of                          z       Pr(Z=z)
the random variable Z = X2 is                            1        20%
                                                         4        15%
and therefore E[X2] is                   8.75            9        35%      #NAME?
                                                        16        30%
Excel also allows us to do this
calculation directly from the
original distribution table:             8.75                              #NAME?

Notice that E[X2] is not the same as ( E[X] )2 .        7.5625             #NAME?

What's the cumulative probability distribution of X ?   x        Pr(X≤x)
                                                        1          20%
                                                        2          35%
                                                        3          70%
                                                        4         100%
                            The LOOKUP function

The function =LOOKUP(value, lookup-range, result-range) looks for "value" in the
"lookup-range," and returns the corresponding entry in the "result-range." It
expects the entries in the lookup-range to be in increasing order, and if the value
isn't found, it selects the last entry in the lookup-range which is less than the
sought-for value.

Here's the IRS 2008 Individual tax table:

       taxable income                          taxes
   from …           to …           paid on excess      plus
       $0.00       $8,025.00            10%               $0.00
   $8,025.00      $32,550.00            15%             $802.50
  $32,550.00      $78,850.00            25%           $4,481.25
  $78,850.00     $164,550.00            28%          $16,056.25
 $164,550.00     $357,700.00            33%          $40,052.25
 $357,700.00                            35%        $103,791.75


   $100,000.00 taxable income

     $16,056.25      #NAME?
     $78,850.00      #NAME?
           28%       #NAME?

     $21,978.25 taxes                 #NAME?

The formula in cell B23 looks in the range B13:B18 for the value $100,000. The last
entry less than or equal to this is the fourth, so the fourth entry in E13:E18 is
returned. The formula in cell B24 shows a variant of the LOOKUP function: If the
result-range is not specified, it's assumed to be the same as the lookup-range.
  Simulating a Random Variable, Using its Tabulated Distribution

How could we build a spreadsheet cell which simulates X ?

One way is to use Excel's "lookup" function. We need a table
containing the cumulative distribution of X.
                                                                                         cumulative
     3      simulated value of X                                       distribution       Pr(X≤x)
             #NAME?                                                   x        Pr(X=x)       0%
                                                                      1          20%        20%
                                                                      2          15%        35%
Notice the offset in the two ranges: If, for example,                 3          35%        70%
RAND() takes the value 0.4527, the third value in the                 4          30%       100%
range J9:J12 (35%) is the final entry which is less than
or equal to 0.4527, and therefore the third value in the
range H10:H13 (3) is returned. The same value (3) will
be returned whenever RAND() is at least 0.35 and less
than 0.70, i.e., 35% of the time.

Let's test this simulation.

 is X=1?      is X=2?     is X=3?      is X=4?   estimated E[X]
     0            0           1            0

    $B$23    $C$23    $D$23    $E$23      $B$8 monitored cell
 20.040% 14.974% 35.110% 29.876%        2.7482 mean
 0.400302 0.356818 0.477317 0.457717 1.089366 sample standard deviation
        0        0        0        0         1 minimum
        1        1        1        1         4 maximum
  100,000 100,000 100,000 100,000 100,000 number of simulation runs

   0.248%      0.221%         0.296%    0.284%    0.0068 margin of error (95% confidence)




                                                                  `
                                  Conditional Expectation

What is E[ X | X is even ]?                                                    x   Pr(X=x)
                                                                               1    20%
Given that X is even, the conditional probability distribution                 2    15%
of X is:                                                                       3    35%
                x       Pr(X=x)                                                4    30%
               2        33.33%
               4        66.67%

Therefore, E[ X | X is even ] is         3.3333

What is E[ X | X is odd ]?

Given that X is odd, the conditional probability distribution
of X is:
                x       Pr(X=x)
               1        36.36%
               3        63.64%

Therefore, E[ X | X is odd ] is          2.2727

What is E[X] ?

Directly, it's               2.75

But we can also work it out "case by case."

E[X] = E[ X | X is even ]∙Pr( X is even ) + E[ X | X is odd ]∙Pr( X is odd )

and this yields              2.75
                               Joint Probability Distributions

The joint distribution of two (or more) random variables indicates the likelihood of
the variables taking any particular combination of values.

                                                            X
                                          2           3            5             15
                               10        5%         12%           3%            18%
                 Y             20        2%          7%           8%             9%
                               30       14%         13%           0%             9%
                                                                                          100%
The table indicates, for example, that Pr( X=3, Y=30 ) = 13% .




What is the (marginal) probability distribution of X ?             x        Pr(X=x)
                                                                   2         21%       ########
                                                                   3         32%       ########
The marginal distributions of both X and Y                         5         11%       ########
can, of course, be written at the "margins"                       15         36%       ########
of the joint-distribution table:

                                                            X
                                          2           3             5            15
                               10        5%         12%            3%           18%      38%      ########
                 Y             20        2%          7%            8%            9%      26%      ########
                               30       14%         13%            0%            9%      36%      ########
                                        21%         32%           11%           36%

What is Pr( X = 3 | Y = 10 ) ?

By definition, Pr( X = 3 | Y = 10 ) = Pr( X = 3 and Y = 10 ) / Pr( Y = 10 ) =            31.58% ########

What is the probability distribution of X+Y ?

Z = X+Y is a random variable with the probability distribution:                  z     Pr(Z=z)
                                                                                12        5%      ########
What is E[ X+Y] ?                                                               13       12%      ########
                                                                                15        3%      ########
From the table to the right:              27.13                                 22        2%      ########
Equivalently, since                                                             23        7%      ########
E[ X+Y] = E[X] + E[Y] ,                   27.13                                 25       26%      ########
                                                                                32       14%      ########
                                                                                33       13%      ########
For any random variables X and Y ,                                              35        9%      ########
E[ X+Y ] = E[X] + E[Y] .                                                        45        9%      ########
                                                                                        100%

What is E[ XY ] ?

We could build a table for the distribution of W = XY , and then compute the expected
value from that table. An Excel "array" formula does precisely the same arithmetic
without requiring the intermediate step:         133.700 {=SUM(E24:H24*D25:D27*E25:H27)}

Notice that E[X]∙E[Y] is                        145.134

E[XY] is not necessarily equal to E[X]∙E[Y] !




                                                                                               20
                                                                                               30

                                                                                           Finally, it copied

                                                                                               5%
                                                                                               2%
                                                                                              14%



                                                                                               1.0
                                                                                               0.8
                                                                                               8.4

                                                                                           And it applied the S

                                                                                                         133.70
necessarily 100%




necessarily 100%
As is explained on the next tab, this formula was
entered by typing Ctrl+Shift+Enter.

Excel automatically expanded E24:H24 to

     2               3       5           15
     2               3       5           15
     2               3       5           15

It also expanded D25:D27 to

    10           10          10          10
    20           20          20          20
    30           30          30          30

Finally, it copied

    5%          12%         3%          18%
    2%           7%         8%           9%
   14%          13%         0%           9%

It then did the indicated arithmetic on a cell-by-cell basis:

    1.0          3.6        1.5         27.0
    0.8          4.2        8.0         27.0
    8.4         11.7        0.0         40.5

And it applied the SUM function to this entire array:

              133.700
                                                 Array formulas


     Working with Array Formulas (an optional Excel skill)

The SUMPRODUCT function returns the                 x        Pr( X = x )
sum of the products of matching terms               1            0.2
in two (or more) lists. For example, the            2            0.5
expected value of X, given the probability          5            0.3
distribution at the right, is

        2.7 =SUMPRODUCT(F4:F6,G4:G6)

However, there's a slicker way to do the same calculation:

        2.7 =SUM(F4:F6*G4:G6), entered as an array formula.

Array formulas are very powerful ... and potentially somewhat confusing.

An "array" has some number of rows, and some number of columns. In an
array formula, Excel finds the largest number of rows, R, in any array in the
formula, and the largest number of columns, C. It then expands any array
that is smaller than R-by-C in one of two ways. If R or C is 1, it copies the
single row or column to fill an entire R-by-C array. Otherwise, it adds extra
cells with the value NA (not available).

Then it does all the arithmetic on a cell-by-cell basis. And it
evaluates any formulas that take array inputs using the resulting
array as the input.

                1           2           3
    4           4           8          12
    5           5          10          15
    6           6          12          18               90

All nine cells inside the box are part of the same array,
defined by the single formula:      {=C28:E28*B29:B31}

The formula was entered as an array formula. First, I
highlighted C29:E31. Then I typed =C28:E28*B29:B31, and
entered the formula by pressing Cntrl+Shift+Enter.

What Excel did was determine R = 3 (the number of rows in
B29:B31) and C = 3 (the number of columns in C28:E28).
It expanded B29:B31 into

                     4           4           4
                     5           5           5
                     6           6           6

and C28:E28 into

                     1           2           3
                     1           2           3
                     1           2           3


                                                    Page 9
                                             Array formulas



Then it computed the products of matching cells, and put them
into the space allocated to the array formula.

The formula in F31 is =SUM(C28:E28*B29:B31), again entered
by pressing Cntrl+Shift+Enter.




                                                 Page 10
                         Independent Random Variables

X and Y are independent if knowledge of the value of one doesn't affect your
assessment of the likelihood of the other taking any specific value. In the previous
example, X and Y were not independent.
                                                         X
                                         2          3           5           15
                           10         5.00%     12.00%       3.00%       18.00%      38.00%
               Y           20         2.00%      7.00%       8.00%        9.00%      26.00%
                           30        14.00%     13.00%       0.00%        9.00%      36.00%
                                     21.00%     32.00%      11.00%       36.00%

Show that X and Y , defined by the joint distribution above, are not independent.

It takes only a single example to show this:   Pr( X=2 ) =              21.00%
                                               Pr( X=2 | Y=10) =        13.16%

If X and Y are independent, then Pr( X=x )∙Pr( Y=y ) = Pr( X=x and Y=y ) for
every pair of values x and y .

                                                         U
                                         2          3            5        15
                           10         7.98%     12.16%        4.18%    13.68%       38.00%
                V          20         5.46%      8.32%        2.86%     9.36%       26.00%
                           30         7.56%     11.52%        3.96%    12.96%       36.00%
                                     21.00%     32.00%       11.00%    36.00%

U and V have the same marginal distributions as do X and Y , respectively, but U and V
are independent random variables.

The marginal distributions of X and Y separately convey no information
concerning whether or not X and Y are independent.


Compute E[ UV ] and E[U]∙E[V] .

E[ UV ] =                 145.134
E[U]∙E[V] =               145.134

These two quantities are equal whenever U and V are independent.
But, just because these two amounts are equal, we can't conclude that
U and V are independent!
                                                       S
                                      2           3           5        15
                           10     21.00%       2.21%       0.00%     14.79%         38.00%
                T          20      0.00%      13.07%       2.65%     10.28%         26.00%
                           30      0.00%      16.72%       8.35%     10.93%         36.00%
                                  21.00%      32.00%      11.00%     36.00%

E[ ST ] =                 145.134
E[S]∙E[T] =               145.134
But S and T are certainly not independent.

And, of course, we saw a few tabs ago that

E[ XY ] =                  133.700
E[X]∙E[Y] =                145.134
                                                                            x      Pr(X=x)
From the very first tab (with X having a different distribution):           1       20%
                                                                            2       15%
                                                                            3       35%
Let Y = X , so the joint distribution is                                    4       30%
                                                             X
                                           1            2            3        4
                              1        20.00%        0.00%        0.00%    0.00%   20.00%
                              2         0.00%       15.00%        0.00%    0.00%   15.00%
                 Y
                              3         0.00%        0.00%       35.00%    0.00%   35.00%
                              4         0.00%        0.00%        0.00%   30.00%   30.00%
                                       20.00%       15.00%       35.00%   30.00%

Then (as we've already computed)

E[ XY ] = E[ X2 ] =                        8.7500
E[X]∙E[Y] = ( E[X] )2 =                    7.5625


In summary:

If X and Y are independent, then E[ XY ] = E[X]∙E[Y] .

However, in general, any of E[ XY ] < E[X]∙E[Y] , E[ XY ] = E[X]∙E[Y] , or E[ XY ] > E[X]∙E[Y]
might be true.
                                                Plant expansion


                            Comparing Plant Expansions

The J.R. Ryland Computer company is considering a plant expansion that will enable
the company to begin production of a new computer product. The company’s president
must determine whether to make the expansion a medium- or large-scale project. An
uncertainty is the demand for the new product, which for planning purposes may be low
demand, medium demand, or high demand. The probability estimates for demand are
0.20, 0.50, and 0.30. respectively. Writing the annual profit in $000s, the firm’s planners
have developed the following profit forecast for the medium- and large-scale expansion
projects.
                                       medium-scale               large-scale
                                     expansion profits        expansion profits
                                                   Prob                    Prob
                           low         $50         20%           $0        20%
             demand medium            $150         50%          $100       50%
                          high        $200         30%          $300       30%


Compute the expected value for the profit associated with the two expansion
alternatives. Which decision is preferred if the objective is to maximize
expected profit?
                                                  expansion profits
                      probability                medium         large
                          20%          low          $50           $0
              demand      50%       medium         $150         $100
                          30%          high        $200         $300

Answer:                       expected profit      $145        $140

Note that the profit numbers themselves are, in reality, expectations - the expected
profit (in the face of current uncertainty about future interest rates, energy prices, and
the like) when demand is low, medium, or high - and our actual calculations above are
actually of the form:

E[profit] =     E[ profit | low demand ]∙Pr( low demand )
              + E[ profit } medium demand ]∙Pr( medium demand )
              + E[ profit | high demand ]∙Pr( high demand ) .




                                                    Page 13
                                   It's a Kind of Magic

Hairy Houdini, a hirsute magician, uses his long beard to conceal the machinations
behind his tricks. His final trick is to “magically” create a number of pigeons that he
releases into the air. (Really, they’re hidden in a pouch behind his beard.) Each time he
attempts another release, he knows there’s a 90% he’ll successfully fool the audience.
He figures, on the basis of past experience, that his end-of-show tips will add up to
about $10 for each successful release if he fools the audience every time. However, if he
fails to fool the audience on any release, the show will end immediately, and he’ll only
receive about $6 in tips for each successful release. How many pigeons should he try to
release (i.e., after how many consecutive successes should he stop voluntarily –
equivalently, how many pigeons should he put in the pouch before his act)?

Hint: This is not a "single-period inventory" problem. However, it yields to either a
marginal-analysis approach, or brute force.

The answer is on the next tab.
                         It's a Kind of Magic (answer)

     23 pigeons

  Approach 1 (marginal analysis):                  Approach 2 (brute force):
given k successful releases, expected
                                                      put n pigeons in pouch
marginal profit from trying for one more
                                                          Pr( fail first on
             k                                       k      pigeon k )      profit
             1         $8.60                         1          10.00%        $0.00
             2         $8.20                         2            9.00%       $6.00
             3         $7.80                         3            8.10%      $12.00
             4         $7.40                         4            7.29%      $18.00
             5         $7.00                         5            6.56%      $24.00
             6         $6.60                         6            5.90%      $30.00
             7         $6.20                         7            5.31%      $36.00
             8         $5.80                         8            4.78%      $42.00
             9         $5.40                         9            4.30%      $48.00
            10         $5.00                        10            3.87%      $54.00
            11         $4.60                        11            3.49%      $60.00
            12         $4.20                        12            3.14%      $66.00
            13         $3.80                        13            2.82%      $72.00
            14         $3.40                        14            2.54%      $78.00
            15         $3.00                        15            2.29%      $84.00
            16         $2.60                        16            2.06%      $90.00
            17         $2.20                        17            1.85%      $96.00
            18         $1.80                        18            1.67% $102.00
            19         $1.40                        19            1.50% $108.00
            20         $1.00                        20            1.35% $114.00
            21         $0.60                        21            1.22% $120.00
            22         $0.20                        22            1.09% $126.00
            23        -$0.20                        23            0.98% $132.00
            24        -$0.60                        24            0.89% $230.00
            25        -$1.00                        25            0.80% $230.00
                                                    26            0.72% $230.00
                                                    27            0.65% $230.00
                                                    28            0.58% $230.00
                                                    29            0.52% $230.00
                                                    30            0.47% $230.00
                                                    31            0.42% $230.00
                                                    32            0.38% $230.00
                                                    33            0.34% $230.00
                                                    34            0.31% $230.00
                                                    35            0.28% $230.00
                                                    36            0.25% $230.00
                                                    37            0.23% $230.00
                                                    38            0.20% $230.00
                                                    39            0.18% $230.00
                                                40 or more        1.64% $230.00

                                                   if we wish to examine cases
                                                     involving the release of 40
                                                 pigeons or more, this table must
                                                            be extended.
  if we wish to examine cases
    involving the release of 40
pigeons or more, this table must
           be extended.
n    E[profit]
23    $57.37

n      $57.37
 1      $9.00
 2     $16.74
 3     $23.38
 4     $29.07
 5     $33.92
 6     $38.06
 7     $41.56
 8     $44.53
 9     $47.03
10     $49.12
11     $50.86
12     $52.31
13     $53.49
14     $54.46
15     $55.24
16     $55.85
17     $56.33
18     $56.70
19     $56.97
20     $57.16
21     $57.28
22     $57.35
23     $57.37
24     $57.35
25     $57.30
26     $57.23
27     $57.14
28     $57.04
29     $56.92
30     $56.80
31     $56.67
32     $56.54
33     $56.41
34     $56.28
35     $56.15
36     $56.03
37     $55.91
38     $55.79
39     $55.68
                            Answer via Simulation

                         success on    first failure                         13 time of first failure (and 0 if he succeeds at least 50 tim
release   still going?
                           this try?    on this try                          13 time of first failure (set to 51 if the first 50 releases suc
   1           1               1              0
   2           1               1              0            5                        10              15         20
   3           1               1              0          $50.00                   $100.00         $72.00     $72.00
   4           1               1              0
   5           1               1              0            $G$7                        $H$7          $I$7        $J$7
   6           1               1              0           $33.89                      $48.97       $55.01      $56.80
   7           1               1              0           $20.06                      $39.57       $52.81      $60.29
   8           1               1              0            $0.00                       $0.00        $0.00       $0.00
   9           1               1              0           $50.00                     $100.00      $150.00     $200.00
  10           1               1              0          100,000                     100,000      100,000     100,000
  11           1               1              0
  12           1               1              0                      $0.12                $0.25     $0.33       $0.37
  13           1               0             13
  14           0               1              0
  15           0               1              0                                                         Simulation Results
  16           0               1              0
  17           0               1              0
                                                                            $60.00
  18           0               1              0
  19           0               0              0
  20           0               1              0                             $55.00
  21           0               1              0
                                                          expected profit




  22           0               1              0                             $50.00
  23           0               1              0
  24           0               1              0                             $45.00
  25           0               1              0
  26           0               1              0
                                                                            $40.00
  27           0               1              0
  28           0               1              0
  29           0               1              0                             $35.00
  30           0               1              0
  31           0               1              0                             $30.00
  32           0               0              0                                       0            10          20
  33           0               1              0
                                                                                                        intended number of birds release
  34           0               1              0
  35           0               1              0
  36           0               1              0
  37           0               1              0        The expected profit curve appears to top out somewhere between
  38           0               0              0
  39           0               1              0        We'll see in a while that the expected number of successful releas
  40           0               1              0        first failure is 9.0. Therefore, the expected profit, as the planned nu
  41           0               1              0        grows larger and larger, is heading for $6∙9.0 =
  42           0               1              0        (since, if he plans a large number of releases, he'll just about defin
  43           0               1              0        act with a failure).
  44           0               1              0
  45           0               1              0
  46           0               1              0
  47           0               1              0
48   0   1   0
49   0   1   0
50   0   0   0
nd 0 if he succeeds at least 50 times)
et to 51 if the first 50 releases succeed)

                  25           30          35          40        45        50
                $72.00       $72.00      $72.00      $72.00    $72.00    $72.00

                   $K$7         $L$7       $M$7        $N$7      $O$7      $P$7    monitored cell
                  $56.88      $56.50      $55.87      $55.21    $54.77    $54.46   mean
                  $63.83      $65.25      $64.95      $63.59    $62.37    $61.28   sample standard deviation
                   $0.00       $0.00       $0.00       $0.00     $0.00     $0.00   minimum
                 $250.00     $300.00     $350.00     $400.00   $450.00   $500.00   maximum
                 100,000     100,000     100,000     100,000   100,000   100,000   number of simulation runs

                    $0.40       $0.40        $0.40     $0.39     $0.39     $0.38 margin of error (95% confidence)


imulation Results




                    30           40           50
 ended number of birds released



rs to top out somewhere between 20 and 25 releases.

cted number of successful releases before the
expected profit, as the planned number of releases
ng for $6∙9.0 =                 $54.00
r of releases, he'll just about definitely end his

								
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