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The lower quartile, median and upper quartile are usually denoted by Q_1, Q_2 and Q_3. To find them you must first arrange the data in order of size.
If n is odd, then Q_2 is the \frac{1}{2}(n+1)th value. If n is even the median is halfway between the \frac{1}{2}nth value and the next.
To find Q_1 find the median of the lower half of the data. To find Q_3, find the median of the upper half of the data.
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Glossary

median

the middle value of a set of data that has been arranged in order of magnitude; half the data values are above it and half are below.

quartile

In an ordered set of data 25% are below the first (Q1), 50% are below the second (Q2), 75% are below the third (Q3).

union

The union of two sets A and B is the set containing all the elements of A and B.

Full Glossary List

This question appears in the following syllabi:

SyllabusModuleSectionTopic
AQA A-Level (UK - Pre-2017)S1Summarising dataMeasures of location
AQA AS Maths 2017StatisticsData Presentation and InterpretationMeasures of Central Tendency
AQA AS/A2 Maths 2017StatisticsData Presentation and InterpretationMeasures of Central Tendency
CBSE XI (India)Statistics and ProbabilityStatisticsRange, mean deviation, variance and standard deviation of grouped data
CCEA A-Level (NI)S1Summarising dataMeasures of location
CIE A-Level (UK)S1Summarising dataMeasures of location
L Statistics - DataL3 Central Tendency and VariationMeasures of Central Tendency
Edexcel A-Level (UK - Pre-2017)S1Summarising dataMeasures of location
Edexcel AS Maths 2017StatisticsMeasures of DataMeasures of Central Tendency
Edexcel AS/A2 Maths 2017StatisticsMeasures of DataMeasures of Central Tendency
I.B. Standard Level5Summarising dataMeasures of location
Methods (UK)M14Summarising dataMeasures of location
I.B. (MSSL)6Summarising dataMeasures of location
OCR A-Level (UK - Pre-2017)S1Summarising dataMeasures of location
OCR AS Maths 2017StatisticsCentral Tendency and VariationMeasures of Central Tendency
OCR MEI AS Maths 2017StatisticsCentral Tendency and VariationMeasures of Central Tendency
OCR-MEI A-Level (UK - Pre-2017)S1Summarising dataMeasures of location
Pre-U A-Level (UK)ProbSummarising dataMeasures of location
Scottish (Highers + Advanced)HSSummarising dataMeasures of location
Scottish HighersSSummarising dataMeasures of location
Universal (all site questions)SSummarising dataMeasures of location
WJEC A-Level (Wales)S1Summarising dataMeasures of location