Scatterplot
Also known as: scatter plot, scatter diagram
A scatterplot is a graph that displays paired data as individual points on a coordinate plane, with one variable on each axis. It shows whether two quantitative variables are related and how strong that relationship is.
Each point on a scatterplot represents one observation with two measurements — a student's study hours and test score, a car's weight and fuel economy. The explanatory (independent) variable goes on the x-axis and the response (dependent) variable on the y-axis. Unlike a bar graph or histogram, a scatterplot plots individual cases rather than summarized totals, so patterns in the cloud of points carry the information.
Read a scatterplot in four steps: direction, form, strength, and outliers. Direction is positive when points rise from left to right and negative when they fall. Form is linear if the points follow a straight path, or curved if they bend. Strength describes how tightly the points cluster around that pattern — a tight band is a strong association, a wide scatter is a weak one. Outliers are points that sit far away from the overall pattern and deserve separate attention.
When the pattern is roughly linear, a line of best fit can be drawn through the points and used to predict a y value from an x value. It is essential to remember that a scatterplot shows association, not causation. Ice cream sales and drowning deaths rise together because both increase in summer, not because one causes the other.
Scatterplots appear on almost every standardized test that includes a data section. The SAT problem solving and data analysis section asks you to read values off a scatterplot and interpret the slope and intercept of its line of best fit, and both the Praxis Core math and reading tests include scatterplot interpretation — the reading test pairing it with bubble graphs and other graphics.
Key takeaways
- A scatterplot plots paired observations as points, with the explanatory variable on the x-axis.
- Describe one by its direction, form, strength, and any outliers.
- A line of best fit summarizes a roughly linear pattern and can be used to make predictions.
- A strong association on a scatterplot does not establish that one variable causes the other.