Scatterplots

Scatterplots illustrate the relationship between two measurement variables.  One variable is put on the X axis and the other on the Y axis.  Every case is then plotted against the two axes.  By looking at how the cases are organized against these two variables, we can tell if there is a relationship between the two.  For example, maybe I want to see if there is a relationship between how many kids are in a family and how many cars that family owns.  I gather information on these two variables from 10 different families.

I plot each of these families by kids and cars.  I make a graph with kids on the X axis and cars on the Y axis.  Then I put the first family at X=7 and Y=3 (the reference lines below are only there to show you that I put the family at where the lines cross).

Plotting the other nine families, I get the following scatterplot.

This scatterplot shows us that there is perhaps some sort of relationship between the two variables.  As the number of kids in a family increases, the number of cars tends to go up.  

It looks like there might be a positive relationship between these two variables.  The statistical measures that quantify what you see here are correlation and regression.

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