Distribution Function of a Random Variable
Probability that the values [latex]x_i[/latex] taken by the random variable X will be strictly less than a given value.
F([latex]x_i[/latex]) = P(X ≤ [latex]x_i[/latex])
A distribution function is a step function that is zero for all values [latex]x_i[/latex] less than or equal to the least value of X, and equal to one for all values [latex]x_i[/latex] strictly greater than the greatest value of X.
Example
Consider an honest die with six faces that are identified by the Ace, King, Queen, Jack, and the numbers ten and two from a deck of cards. Consider a random variable defined by the function X from the set Ω of possible results on a set E of values attributed to these faces such as E = {100, 50, 20, 10, 5, 1}, as represented in this table:| Result (A) | P(A) | X(A) |
| Ace | [latex]\frac{1}{6}[/latex] | 100 |
| King | [latex]\frac{1}{6}[/latex] | 50 |
| Queen | [latex]\frac{1}{6}[/latex] | 20 |
| Jack | [latex]\frac{1}{6}[/latex] | 10 |
| Ten | [latex]\frac{1}{6}[/latex] | 5 |
| Two | [latex]\frac{1}{6}[/latex] | 1 |
| [latex]x_i[/latex] | F([latex]x_i[/latex]) = P(X < [latex]x_i[/latex]) |
| [latex]x_i[/latex] ≤ 1 | 0 |
| 1 < [latex]x_i[/latex] ≤ 5 | [latex]\frac{1}{6}[/latex] |
| 5 < [latex]x_i[/latex] ≤ 10 | [latex]\frac{1}{6}+\frac{1}{6}=\frac{1}{3}[/latex] |
| 10 < [latex]x_i[/latex] ≤ 20 | [latex]\frac{1}{6}+\frac{1}{6}+\frac{1}{6}=\frac{1}{2}[/latex] |
| 20 < [latex]x_i[/latex] ≤ 50 | [latex]\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}=\frac{2}{3}[/latex] |
| 50 < [latex]x_i[/latex] ≤ 100 | [latex]\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}=\frac{5}{6}[/latex] |
| 100 < [latex]x_i[/latex] | [latex]\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}+\frac{1}{6}= 1[/latex] |
