A class graph describes a group. State what the bars show, use the counts, and leave out any reason for one student’s value, because the graph cannot give it.
This lesson opens the sub-cluster on health-themed science without medical advice. It applies the reading skills from earlier data lessons.
How do I read a grouped bar chart?
Read the axes, add the bars to find the class total, then describe the tallest bar and the spread. Do these three things before forming any statement.
Worked example: resting pulse of a class
An anonymised chart shows the resting pulse of 30 students in beats per minute, grouped in bands.
| Band (beats per minute) | 60–69 | 70–79 | 80–89 | 90–99 | 100–109 |
|---|---|---|---|---|---|
| Number of students | 4 | 12 | 9 | 4 | 1 |
Check the total. 4 + 12 + 9 + 4 + 1 = 30, matching the class size.
Most common band. 70–79, with 12 students. As a fraction, 12 ÷ 30 = 0.4, or 40% of the class.
Students at 80 or above. 9 + 4 + 1 = 14, which is 14 ÷ 30 = 46.7% of the class.
Model answer. The most common band is 70–79, with 12 out of 30 students. The number of students falls band by band above 79, and only one student is in the 100–109 band.
The last point describes the bar without explaining it. The graph cannot tell us why that student’s value is high.
The mistake that costs marks
Students build a story around the tall or isolated bar. The story feels natural, but it is a guess.
| Sentence | Verdict |
|---|---|
| “The student in 100–109 is unwell.” | Unsafe: a diagnosis with no evidence |
| “The student in 100–109 just ran up the stairs.” | Unsupported: a cause the graph does not give |
| “One student is in 100–109, the highest band.” | Exam-ready: describes the bar |
If asked why, write that the graph does not show the cause and that more information would be needed, such as whether readings were taken at rest.
Check yourself
Using the table above, how many students have a resting pulse below 80, and what fraction of the class is that?
Answer
Below 80 means the first two bands: 4 + 12 = 16 students.
Fraction: 16 ÷ 30 = 8/15, or about 53.3%. Check: 16 + 14 = 30, using the 14 students at 80 or above.
What to study next
Go on to distinguishing a biological mechanism from a treatment recommendation. The graph evidence comparison lab gives extra graph-reading practice.
For a teacher to work through graph questions with you, see online one-to-one Science tuition.