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What you need to know
What you need to know:
We may choose to calculate the range of a set of data to determine how spread out it is, however the range is not a particularly good measure, so we might instead use the interquartile range. The interquartile range is found using similar methods to when you find the median – rather than lining your data up and cutting it in half to find the half-way point, you cut your data into 4 equal parts and find the quartiles – points that are one quarter of the way along when all the data is lined up. They are, in order: the lower quartile, Q_1, the median, Q_2, and the upper quartile, Q_3. You should be able to find to these quartiles, and then find the interquartile range by subtracting Q_1 from Q_3.
Once you have these values, you may be asked to draw a boxplot to display them. A boxplot is a visual representation of the data which displays Q_1, Q_2, Q_3,, the lowest value, and the highest value, along with a number line underneath the whole picture. An example of the format for drawing a boxplot is, is shown below.
A common type of question involves you being asked to compare two sets of data by looking at their box plot. This usually amounts to saying how one has a higher average (median) than the other, and one is more spread out (has a bigger range/interquartile range) than the other.
Box Plots questions, Box Plot revision and practice tests are all available on this GCSE Maths revision page. The Box Plots resources have been designed with students, teachers and tutors in mind to give people a great selection of resources. For more exceptional GCSE Maths resources please visit our main GCSE Maths revision page.
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