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Averages from Tables (Edexcel GCSE Maths)
Revision Note
Averages from Tables & Charts
How do we find averages if there are lots of values?
- In reality there will be far more data to work with than just a few numbers
- In these cases the data is usually organised in such a way to make it easier to follow and understand
- For example in a table or chart
- We can still find the mean, median and mode but have to ensure we understand what the table or chart is telling us
How do I find averages from a table or chart?
- Finding the median and mode from tables/charts is fairly straightforward once you understand what the table/chart is telling you
- Tables allow data to be summarised neatly
- and quite importantly, they put the data into order
Finding the mean from (discrete) data presented in tables
- The mean can be found as you long as understand what a table is telling you
- Tables tell us the data value
- e.g. the number of pets per household
- and the frequency of that data value
- e.g. the number of households with that number of pets
- Tables tell us the data value
STEP 1
Add a column to the table and work out "data value" × "frequency"
(This is effectively doing the 'adding up' part of finding the mean in stages)
STEP 2
Find the total of the extra column to give the overall total of the data values
STEP 3
Find the mean by dividing this total by the total of the frequency column
i.e. divide the total of the data values by the number of data values
Finding the median from (discrete) data presented in tables
- The median is the middle value when the data is in order
- The position of the median can be found by using , where is the number of data values
- Use the table to deduce where the value lies
- e.g. if the median is the 7th value and the frequency of the first two rows are 4 and 7
- the median will be one of the 7 values the second row of that table
- e.g. if the median is the 7th value and the frequency of the first two rows are 4 and 7
Finding the mode (or modal value)
- The mode (or modal value) is simple to identify
- Look for the highest frequency
- and thus find the corresponding data value
- Make sure you do not confuse the data value with the frequency!
- The frequency (in a table) tells us the row the mode is in
- Look for the highest frequency
Worked example
The bar chart shows data about the shoe sizes of pupils in class 11A.
Find the mean shoe size for the class,
Although the data is given in a bar chart, this is essentially the same as a table.
Rewrite it as a table but add an extra column to help find the total of all the shoe sizes.
Shoe size (x ) | Frequency (f ) | xf |
6 | 1 | 6 × 1 = 6 |
6.5 | 1 | 6.5 × 1 = 6.5 |
7 | 3 | 7 × 3 = 21 |
7.5 | 2 | 7.5 × 2 = 15 |
8 | 4 | 8 × 4 = 32 |
9 | 6 | 9 × 6 = 54 |
10 | 11 | 10 × 11 = 110 |
11 | 2 | 11 × 2 = 22 |
12 | 1 | 12 × 1 = 12 |
Total | 31 | 278.5 |
Mean
Mean = 8.98 (3 s.f.)
Note that the mean does not have to be an actual shoe size.
Find the median shoe size,
The bar chart/table has the data in order already so find the position of the median.
The median is the 16th value.
There are 1 + 1 + 3 + 2 + 4 = 11 values in the first five rows of the table.
There are 11 + 6 = 17 values in the first six rows of the table.
Therefore the 16th value must be in the sixth row.
Median shoe size is 9
Suggest a reason the shop owner may wish to know the modal shoe size of their customers.
A shop owner would want to know the modal shoe size of their customers as this size will be more likely to sell than other sizes so the shop owner should order more shoes in the modal size to stock the shop with
Averages from Grouped Data
What is grouped data and why use it?
- Some data for a particular scenario can vary a lot
- For example the heights of people
- particularly if you include a mixture of children and adults
- For example the heights of people
- Data like height is also continuous (data that can be measured)
- Such data is difficult, even using a table, to list every value
- There is also little difference between someone who is, say, 176 cm tall and someone who is 177cm tall
- So we often group data into classes but that leads to one crucial point
- When data is grouped, we lose the raw data
- With height, we know that 10 people have a height between 150 cm and 160 cm
- but we won't know exactly what those 10 heights are
- This means we cannot find the actual mean, median and mode from grouped data by their original definitions
- but we can estimate the mean
- look for the word estimate in questions - it's a big clue to use the method in this note!
- we can talk about the class interval that the median lies in
- and we can talk about the modal class (the class interval the mode lies in)
- but we can estimate the mean
How do I find/estimate the mean from grouped data?
- There is one extra stage to this method compared to finding the mean from tables with discrete data
- We use the class midpoints as our data values
- For example, if heights are split into class intervals 150 ≤ x < 160, 160 ≤ x < 170, etc
- the midpoints, and so data values, would be 155, 165, etc
STEP 1
Draw an extra two columns on the end of a table of the grouped data
In the first new column write down the midpoint of each class interval
STEP 2
Work out "frequency" × "midpoint" (This is often called fx )
STEP 3
Total the fx column, and if not already done nor mentioned in the question, total the frequency column to find the number of data values involved
STEP 4
Estimate the mean by using its formula; "total of fx" ÷ "no. of data values"
- Be careful with midpoints
- not all class intervals will be of equal size
- so there may not be a nice pattern to the midpoints
How do I find the class interval that the median lies in?
- Find the position of the median using , where is the number of data values (total of the frequency column)
- Use the table to deduce the class interval containing the value
- e.g. if the median is the 7th value and the frequency of the first two class intervals are 4 and 7
- the median will lie in the second class interval of the table
- e.g. if the median is the 7th value and the frequency of the first two class intervals are 4 and 7
- Note the language used when working with the median for grouped data
- Rather than 'the median' we refer to the 'class interval containing the median'
How do I find the modal class interval?
- It is impossible to find the modal value (i.e. mode) for grouped data, because we don't have access to the original data values
- i.e. it is impossible to say which value occurs the most times
- For grouped data we talk about the modal class instead
- This is the class with the highest frequency
- Just find the highest frequency in the table
- The corresponding class interval tells you the modal class
- This is the class with the highest frequency
- Be careful not to confuse the modal class with its frequency
- e.g. if the highest frequency in the table is 34, corresponding to the class interval
- then the modal class is
- The modal class is NOT '34'!
- e.g. if the highest frequency in the table is 34, corresponding to the class interval
Examiner Tip
- When presented with data in a table it may not be obvious whether the data is grouped or not
- when you see the phrase “estimate the mean” you know that you are in the world of grouped data
- so use the midpoint technique to answer the question
Worked example
The weights of 20 three-week-old Labrador puppies were recorded at a vet's clinic.
The results are shown in the table below.
Weight, w kg | Frequency |
3 ≤ w < 3.5 | 3 |
3.5 ≤ w < 4 | 4 |
4 ≤ w < 4.5 | 6 |
4.5 ≤ w < 5 | 5 |
5 ≤ w < 6 | 2 |
Estimate the mean weight of these puppies.
First add two columns to the table and complete the first new column with the midpoints of the class intervals.
Complete the second extra column by calculating "fx".
A total row would also be useful.
Weight, w kg | Frequency | Midpoint | "fx" |
3 ≤ w < 3.5 | 3 | 3.25 | 3 × 3.25 = 9.75 |
3.5 ≤ w < 4 | 4 | 3.75 | 4 × 3.75 = 15 |
4 ≤ w < 4.5 | 6 | 4.25 | 6 × 4.25 = 25.5 |
4.5 ≤ w < 5 | 5 | 4.75 | 5 × 4.75 = 23.75 |
5 ≤ w < 6 | 2 | 5.5 | 2 × 5.5 = 11 |
Total | 20 | 85 |
Now we can find the mean.
Mean
An estimate of the mean weight of the puppies is 4.25 kg
Write down the modal class.
Looking for the highest frequency in the table we can see it is 6.
This corresponds to the interval 4 ≤ w < 4.5.
The modal class is 4 ≤ w < 4.5
A common error is to write down 6 as the mode (modal value).
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