What Are Bivariate Data?
Data that involves two variables measured on the same set of subjects or items. We collect bivariate data to investigate whether a relationship exists between the two variables.
In science, we often want to know if one thing affects another. For example:
- Does the height of a plant depend on the amount of sunlight it receives?
- Does the temperature of water affect how quickly sugar dissolves?
- Is there a link between a person's age and their resting heart rate?
In each case, we're measuring two quantities for every individual or trial. That's what makes the data bivariate ("bi" = two, "variate" = variable).
In a scientific investigation, one variable is usually the independent variable (the one you change or select) and the other is the dependent variable (the one you measure as a result). When we plot bivariate data, the independent variable goes on the x-axis and the dependent variable goes on the y-axis.
Think of bivariate data like a class register that records two pieces of information for every student — say, the number of hours they studied and their test score. Each student gives you a pair of numbers, not just one.
Scatter Graphs (Scatter Plots)
A type of graph that displays bivariate data as individual points on a coordinate grid. Each point represents one pair of measurements .
How to draw a scatter graph:
- Draw and label the x-axis with the independent variable and the y-axis with the dependent variable.
- Choose appropriate, evenly spaced scales so all data fit on the graph.
- Plot each data pair as a single dot or cross at the correct position.
- Give your graph a title that describes what is being compared.
- Do not join the dots — scatter graphs show individual data points, not a continuous line.
Always start your axis at zero unless the data values are all very large (e.g., years 2010–2025). If you break an axis, show it with a zig-zag line near the origin.
Example: A student measures the mass of a backpack and the time it takes different people to run 100 m.
| Mass of backpack (kg) | Time for 100 m (s) |
|---|---|
| 0 | 14.2 |
| 2 | 14.5 |
| 4 | 15.1 |
| 6 | 15.8 |
| 8 | 16.6 |
| 10 | 17.3 |
Plot mass on the x-axis and time on the y-axis. Each row becomes a point: , , and so on.
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