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Invite three children of varying heights to stand in front of the board.
Who is the middle? Bring another child into the line and repeat the
question. Pupils are likely to say that there is no middle one, as there
are now four children!
Return to the three original children. Measure and mark their heights
on the board. Ask, "Is the middle person's height exactly half
way between the tallest and shortest?" The pupils should realise
that the middle person's height is not the middle amount. Explain, "The
person in the middle is the median." Average and mean are the same.The
median may be different.
Add all the heights and share them equally. In other words, find the
total and divide by the number of people. Compare the result to the
median height.
Repeat the exercise using 6 children.Give four children
a handful of cubes. As a whole class, work out the total and average
amount. Set out the data on a spreadsheet:
Check that children understand that, to find the mean, we divide the
total number of cubes by the number of children. The formula in D5 may
be entered as: =C5/4
Give out the task sheets to do Part 1.
Bring the class together and review the task. Write the
first example on the board or demonstration page, or enter directly
into the spreadsheet:
Big difference!!! We are now going to work out the mean in one step
- i.e. without working out the total first.
What is the formula in E1? Pupils may suggest,
=A1+B1+C1+D1/4
Try this. Is it the result you expected? What has the computer done?
Try:
=(A1+B1+C1+D1)/4
Is this what you expected? Why are the brackets needed? Remember, you
have to find the total first.
Discuss another example, then ask the pupils to do the second exercise
on the task sheet.
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