Probability Distribution Table

How to find the expected value, variance, and standard deviation from the probability distribution table: formula, 4 examples, and their solutions.

Definition

Definition

This is a probability distribution table.

X: Value for each case
P(X): Probability for each case

Formula

Formula

These are the formulas
to find E(X), V(X), and σ(X).

p1 + p2 + p3 = 1
The sum of the probabilities is 1.

E(X) = x1p1 + x2p2 + x3p3 + ...
Expected Value

V(X) = E(X2) - {E(X)}2
New formula to find the variance V(X)

σ(X) = √V(X)
Standard Deviation

Example 1

Example

Solution

The sum of the probabilities is 1.

So
1/4 + a + 1/3 + 1/6 = 1.

Multiply 12 to both sides.

[1/4]⋅12 = 3
+a⋅12 = +12a
+[1/3]⋅12 = +4
+[1/6]⋅12 = +2

1⋅12 = 12

3 + 4 + 2
= 7 + 2
= 9

Move 9 to the right side.

Then 12a = 3.

Divide both sides by 3.

Then 4a = 1.

Divide both sides by 4.

Then a = 1/4.

So a = 1/4.

Example 2

Example

Solution

You just found that
a = 1/4.

To find the expected value E(X),
multiply X and P(X) for each case,
and add the products.

So
E(X) = (-1)⋅[1/4] + 1⋅[1/4] + 2⋅[1/3] + 3⋅[1/6].

Cancel (-1)⋅[1/4] and +1⋅[1/4].

2⋅[1/3] = 2/3
+3⋅[1/6] = +3/6

2/3 = 4/6

4/6 + 3/6 = 7/6

So E(X) = 7/6.

Example 3

Example

Solution

You found that
a = 1/4 and E(X) = 7/6.

Then find E(X2).

To find E(X2),
first find X2.

Square X values.

(-1)2 = 1
12 = 1
22 = 4
32 = 9

Find E(X2).

Multiply X2 and P(X) for each case,
and add the products.

So
E(X2) = 1⋅[1/4] + 1⋅[1/4] + 4⋅[1/3] + 9⋅[1/6].

1⋅[1/4] = 1/4
+1⋅[1/4] = +1/4
+4⋅[1/3] = +4/3
+9⋅[1/6] = +9/6 = +3/2

The least common multiple of the denominators,
4, 3, 2,
is 12.

So, to add and subtract these fractions,
change the denominators to 12.

[1/4]⋅[3/3] = 3/12
+[1/4]⋅[3/3] = +3/12
+[4/3]⋅[4/4] = +16/12
+[3/2]⋅[6/6] = +18/12

3 + 3 = 6
16 + 18 = 34








6 + 34 = 40

Reduce 40 to, 40/4, 10
and reduce 12 to, 12/4, 3.

E(X) = 7/6
E(X2) = 10/3

So
V(X) = 10/3 - (7/6)2.

-(7/6)2
= -[72/62]
= -[49/36]

10/3 = 120/36

120 - 49 = 71

So V(X) = 71/36.

Example 4

Example

Solution

You found that
V(X) = 71/36.

Then σ(X) = √71/36.

71/36 = √71/√36

Divide Radicals

36
= √62
= 6

Square Root

So σ(X) = √71/6.