What Are Box Plots?
A box plot (also called a box-and-whisker diagram) is a visual way to display the spread and distribution of a data set using five key values. It gives you an instant picture of where most of the data sits, how spread out it is, and whether there are any unusual values.
Box plots are especially useful when you want to compare two or more data sets side by side — for example, comparing reaction times between two experimental groups, the growth of plants under different conditions, or test scores between two classes.
In MYP Sciences, box plots are a powerful tool for analysing experimental data. When you collect measurements in an investigation — such as the temperature of a solution over time or the heights of seedlings — a box plot helps you summarise and communicate your findings clearly.
A statistical diagram that displays the minimum, lower quartile (Q1), median (Q2), upper quartile (Q3), and maximum of a data set using a rectangular box and two whiskers.
The Five-Number Summary
Every box plot is built from a five-number summary. Before you can draw a box plot, you need to find these five values:
- Minimum — the smallest value in the data set
- Lower Quartile (Q1) — the value below which 25% of the data falls
- Median (Q2) — the middle value; 50% of data is below this
- Upper Quartile (Q3) — the value below which 75% of the data falls
- Maximum — the largest value in the data set
A set of five descriptive statistics — minimum, Q1 (lower quartile), Q2 (median), Q3 (upper quartile), and maximum — that summarise the spread and centre of a data set.
Think of the five-number summary like cutting a queue of people into four equal groups. Q1 is where the first quarter ends, the median is the halfway point, and Q3 is where three-quarters of the queue has passed. The minimum and maximum are simply the first and last person in the queue.
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