Lesson - Quartiles

Learning Intention: define quartiles and demonstrate use of quartile ranges for analysing data.

Diagnostic:

Explanation:

Quartiles are the values that divide an ordered data set into four (approximately) equal parts.

There are three quartiles:

1. The lower quartile (Q1) divides off (approximately) the lower 25% of data values.
2. The second quartile (Q2) is the median.
3. The upper quartile, (Q3) divides off (approximately) the upper 25% of data values.

Inter-Quartile range

When there are outliers in the data, it is more appropriate to calculate the interquartile range.

• The interquartile range is the range of the middle half of the data.
• interquartile range = upper quartile – lower quartile
• E.g. 7 - 4 = 3

We can show the quartiles and inter quartile range on a box and whisker plot:

Example:

Tasks:

1) Using an analysis you have completed previously, calculate the upper and lower quartiles and draw a box and whisker plot over the top of your box plot.

Diagnostic answers: