# Frequency distribution of quantitative data

In the lesson about raw data, we have the following table for the number of smartphones owned per family.

For such quantitative data, it is quite straightforward to make a frequency distribution table.

People either own 1 , 2, 3, 4, or 5 smartphones. Then, all you need to do is to find the frequency of 1, 2, 3, 4, and 5.

The frequency of 1 is 5.

The frequency of 2 is 20.

The frequency of 3 is 15

The frequency of 4 is 8

The frequency of 5 is 2

Finally, just put the information in a table.

Number of phones |
frequency (f) |

1 |
5 |

2 |
20 |

3 |
15 |

4 |
8 |

5 | 2 |

As you can see, there 5 groups (1, 2, 3, 4, and 5). To be more precise we prefer to say 5 classes.

Group is the name that we use for qualitative data.

It is not wrong to have less than 5 classes. However, usually the least number of classes is 5 and we like to use intervals for those classes.

Pretend you want to organize the data using only 3 classes. As you can see in the table below, we now have intervals for those classes. The first interval is 1-2.

The only problem is that when you use intervals, we cannot get at least 5 classes for this set of data.

Again, the classes must be intervals. However, if it is hard to get at least 5 classes, it may be best to create a frequency distribution using only** single-valued classes**.

For single-valued classes, we can construct a bar graph to display the data.

Number of phones |
frequency (f) |

1 -2 |
25 |

3-4 | 23 |

5-6 | 2 |

Although we don’t usually have frequency tables with just 3 classes, I will use the table to give you some important definitions.

**Lower class limits **

1, 3, and 5 are called lower class limits.

**Upper class limits**

2, 4, and 6 are called upper class limits.

3-4 is called the second class. The upper limit of the third class is 6.

**Class boundary**

Class boundary is the midpoint between the upper class limit of one class and the lower class limit of the next class.

For example, the class boundaries of the second class are 2.5 and 4.5.

4.5 is the midpoint between 4 and 5.

2.5 is the midpoint between 2 and 3.

We write 2.5 to less than 4.5

We call 2.5 he lower boundary and 4.5 the upper boundary.

**Class width**

Class width = Upper boundary – Lower boundary

For this example, class width = 4.5 – 2.5 = 2.

**Class midpoint**

The class midpoint, also called mark, is found by using the following formula.

For our example, = 1.5

In the next lesson, we will outline the steps for constructing a frequency distribution table using intervals only for the classes.

Procedure for constructing a frequency table