In the UK coins come as 1p,2p, 5p, 10p, 20p, 50p, £1 or £2 coins. Suppose you have 5 1p coins,3 2p coins and 2 5p coins. You select three coins and are only interested in the total value of the selection. To find the sampling distribution X for the value of the selection, we must list all the possible values of the selection, along with the probability of each one.
|
Selection/p |
Value/p X |
Probability, P(X=x) |
|
1, 1, 1 |
3 |
|
|
1, 1, 2 or 1, 2, 1 or 2, 1, 1 |
4 |
|
|
1, 2, 2 or 2, 1, 2 or 2, 2, 1 |
5 |
|
|
2, 2, 2 |
6 |
|
|
1, 1, 5 or 1, 5, 1 or 5, 1, 1 |
7 |
|
|
1, 2, 5 or 1, 5, 2 or 2, 1, 5 or 2, 5, 1 or 5, 1, 2or 5, 2, 1 |
8 |
|
|
2, 2, 5 or 2, 5, 2 or 5, 2, 2 |
9 |
|
|
1, 5, 5 or 5, 1, 5 or 5, 5, 1 |
11 |
|
|
2, 5, 5 or 5, 2, 5 or 5, 5, 2 |
12 |
|
We can find the expected value of the selection using the formula
and the variance is
The variance is then