Choose and draw the right graph
Select a display for the data type and plot clear, labelled graphs with suitable scales and defensible lines.
Cambridge International Biology 9700, 2028–2030. Paper 3 assesses practical skills in a supervised laboratory; these lessons support preparation but do not replace safe, supervised laboratory experience.
Before you begin
- Use biological vocabulary and basic calculations from the AS Biology course. Follow your school laboratory rules and teacher instructions for all practical work.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which display is generally appropriate for continuous independent and dependent variables?
What you will learn
- Choose a line graph, bar chart or histogram for the data.
- Place variables on correct axes and include units.
- Use scales, points and lines that represent the observed results accurately.
Match display to data
Use a graph for continuous quantitative variables, a bar chart for discontinuous or categorical data, and a histogram for grouped frequency data. Put the independent variable on the x-axis and dependent variable on the y-axis.
Axis labels should match table headings and include units. Choose scales that use most of the available grid and make the values easy to read.
Plot and connect honestly
Mark points accurately with small crosses or dots. Use a line of best fit when the pattern supports it, a smooth curve when biologically appropriate, or joined straight lines when the task requires that representation.
Do not extrapolate beyond the measured range unless the trend justifies it. A graph should reveal the data rather than suggest certainty the measurements do not support.
Pause and explain
A graph of enzyme rate against temperature should put which variable on the x-axis?
Worked example
A student measures mean growth at five continuous temperatures. Which display is appropriate and how should it be labelled?
Show the worked solution
- Use a graph because temperature and mean growth are quantitative and continuous.
- Put temperature on the x-axis and mean growth on the y-axis.
- Include units and plot a suitable trend line.
Answer Use a graph with temperature / °C on the x-axis and mean growth / mm on the y-axis. Plot points accurately and draw a line of best fit appropriate to the pattern.
From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
You have frequency counts in four colour categories. Which display should you choose?
Give me a hint
Identify whether the x-values are continuous or categories.
Compare my reasoning
- The categories are discontinuous.
- A bar chart displays separate category counts clearly.
Use a bar chart because colour categories are discontinuous and categorical.
Look for these in your work
- Data type identified
- Display justified
A graph uses 10% of the available grid and labels axes “x” and “y”. Give three improvements.
Give me a hint
Make data easier to read and connect axes to measured quantities.
Compare my reasoning
- Choose a scale that uses most of the grid.
- Label axes with variable names and units.
- Plot marks accurately and select a defensible line or curve.
Use most of the grid, label each axis with the measured quantity and unit, and accurately plot points with a line or curve supported by the data.
Look for these in your work
- Scale improved
- Axes labelled
- Data and line quality addressed
The measured relationship rises between 1 and 5 units of a variable. Can a line automatically be extended to 10 units?
Give me a hint
Separate interpolation from extrapolation.
Compare my reasoning
- The data only cover 1–5.
- Extension beyond the measured range needs a justified assumption or new data.
No. Do not extrapolate to 10 unless the trend can reasonably be assumed to continue; otherwise collect more data.
Look for these in your work
- Extrapolation recognised
- Evidence limit explained
Common mistakes
- Using bars for a continuous variable without justification.
- Omitting units or swapping the independent and dependent axes.
- Drawing a best-fit line through every point or extrapolating automatically.
What can you explain now?
Which display is generally appropriate for continuous independent and dependent variables?
Compare with the explanation
A graph with the independent variable on x
This shows the relationship across a continuous range.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
When do you use a graph, a bar chart and a histogram?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.