Scatter plots
Two things were measured on each individual. How can one picture show how the two go together?
Where you are 2 inside
What this area is about
A scatter plot shows two numerical variables recorded on the same individuals. Each individual becomes one point (x, y) on the coordinate plane, across by one measurement and up by the other, so the two numbers that belong together stay together. The whole cloud of points then shows whether, and how, the variables are associated. An association on a plot does not by itself show that one variable affects the other.
How these fit together
A procedure. Plotting paired data turns each row of a table into one point, with the explanatory variable across and the response variable up when one is meant to explain the other.
A description. Describing a scatter plot reads the finished cloud: its direction (positive, negative or none), its form, its strength, and any clusters or outliers, all stated in the variables’ own units and context.
A path through it
Plot the points first, then describe the cloud they make.
Finish here and you can measure a linear association with correlation, draw a line of best fit through the cloud, and meet the least-squares criterion, one exact rule for which line fits best.