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How to Find the Averages and Range From Grouped Data

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I am a former maths teacher and owner of DoingMaths. I love writing about maths, its applications, and fun mathematical facts.

Grouped Frequency Table Showing Customers in a Shop

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Finding the Mean, Median and Mode From Grouped Frequency Tables

In previous articles, we have had a look at how to find the mean, median, mode and range of a group of data when that data was presented as a list of numbers and also when it was presented in frequency tables, but what about when we have grouped data?

When collecting information over a large range such as the time taken to complete a task or the age of people visiting a museum, it generally makes sense to record our data in groups. For example, when collecting data on age, instead of tallying all of the one year olds in one row, two year olds in the next row etc. we may choose to tally in groups of a set number of years e.g 0 ≤ age < 10, 10 ≤ age < 20 and so on. This makes it easier to record and a lot easier to analyse afterwards. Imagine drawing a pie chart of this data if we had a separate slice for each age; not a good idea.

However, this leads us to a potential problem. By recording our data in groups, we lose a degree of accuracy that would be present if we tallied each item precisely. How do we find the mean if we don't know the exact figure for each item?

In this case, we can find estimates for the averages and range.

Revise How to Find the Mean, Median, Mode and Range

The Ages of People Visiting a Museum One Morning

AgeFrequency

0 ≤ age < 20

17

20 ≤ age < 30

8

30 ≤ age < 40

6

40 ≤ age < 50

2

50 ≤ age < 60

14

60 ≤ age < 70

10

70 ≤ age < 80

4

80 ≤ age < 100

0

The First Step: Finding the Mid-Point of Each Group

As we need exact values in order to find the mean, our first step is to assume that for each group, the values average out to the mid-point of that group. In our example above, the mid-point of the first group is half-way between 0 and 20, so 10. The mid-point of the group 20 ≤ age < 30 is 25 and so on.

If you are unsure of the mid-point, simply add the two end values together and then divide by 2.

We will now add this to our table.

Our Updated Frequency Table With Mid-Points Column

AgeFrequencyMid-point

0 ≤ age < 20

17

10

20 ≤ age < 30

8

25

30 ≤ age < 40

6

35

40 ≤ age < 50

2

45

50 ≤ age < 60

14

55

60 ≤ age < 70

10

65

70 ≤ age < 80

4

75

80 ≤ age < 100

0

90

Finding an Estimate for the Mean From a Grouped Frequency Table

To find the mean, we now use the same method as for an un-grouped frequency table but use out mid-points. So for each row, we multiply the mid-point by the frequency. We then add these new values together before dividing by the total frequency to give us an estimate for the mean.

AgeFrequencyMid-pointMid-point x Frequency

0 ≤ age < 20

17

10

170

20 ≤ age < 30

8

25

200

30 ≤ age < 40

6

35

210

40 ≤ age < 50

2

45

90

50 ≤ age < 60

14

55

770

60 ≤ age < 70

10

65

650

70 ≤ age < 80

4

75

300

80 ≤ age < 100

0

90

0

Total

61

2390

Our estimate for the mean is 2390 ÷ 61 = 39.2 (to 1 d.p.).

Finding the Modal Group

When dealing with data in a frequency table, we can't find an actual mode as we don't know the exact values. Instead we look for the group with the highest frequency and call this the modal group.

In our example above, there are more people in the 0 ≤ age < 20 group than in any other group, so this is our modal group.

Finding the Median From a Grouped Frequency Table

Finding the median can be approached in two different ways when dealing with grouped data. We are just going to look at the simplest method here which is to find the group which contains the middle value and state this. This method is perfectly good for comparing data sets and is all that is required at GCSE level for example.

To find the middle value we add one to the total frequency and divide by 2. In our example above, this gives us (61 + 1) ÷ 2 = 31, and so the median is the 31st value.

By adding a cumulative frequency column to our table, we can see that this is the last value in the 30 ≤ age < 40 group. We now say that the median is contained in this group.

AgeFrequencyCumulative Frequency

0 ≤ age < 20

17

17

20 ≤ age < 30

8

25

30 ≤ age < 40

6

31

40 ≤ age < 50

2

33

50 ≤ age < 60

14

47

60 ≤ age < 70

10

57

70 ≤ age < 80

4

61

80 ≤ age < 100

0

61

Finding the Range From a Grouped Frequency Table

To find the range from a grouped frequency table, we look for the largest and smallest possible values. In our example, the largest possible value is the top of the oldest group, 100, while the smallest possible value is the bottom of the youngest group and so is 0.

The range is therefore 100 − 0 = 100.

Note, that just like the mean, this is an estimate as we can't know for certain what the largest and smallest values actually are.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 David