1. The smallest and largest value for the set is determined:

Minimum amount: 5.1

Maximum amount: 14.9

2. Define the lower bound of the first class (Li) which shall be equal to or slightly lower than the lowest value of observations:

LI: 5.1

3. Define the upper bound of the last class (Ls) that should be equal to or slightly higher than the largest value of observations:

LS: 15

4. Define the number of classes (K), which will be calculated using K = _{ } . It must be between 5 and 20. In this case, K equals approximately 8,94.

5. Known the number of classes defines the amplitude of each class:

_{ }

In the example, a will be equal to:

_{ }

6. With the knowledge of the amplitude of each class, the limits are defined for each class (lower and upper), where Lower limit will be 5.1 and the upper limit will be 15 + 1.23.

7.

Class Interval | Absolute Frequency | Cumulative frequency | Relative Frequency |

| 13 | 13 | 16,25% |

| 21 | 34 | 26,25% |

| 22 | 56 | 27,50% |

| 15 | 71 | 18,75% |

| 4 | 75 | 5,00% |

| 3 | 78 | 3,75% |

| 1 | 79 | 1,25% |

| 1 | 80 | 1,25% |

80 | 100% |