Scatter diagram

The scatter diagram graphs pairs of numerical data, with one variable on each axis, to look for a relationship between them. If the variables are correlated, the points will fall along a line or curve. The better the correlation, the tighter the points will hug the line.

When to Use a Scatter Diagram

• When trying to determine whether the two variables are related, such as…
  • When trying to identify potential root causes of problems.
  • After brainstorming causes and effects using a fishbone diagram, to determine objectively whether a particular cause and effect are related.
  • When determining whether two effects that appear to be related both occur with the same cause.

  Scatter Diagram Procedure

  Collect pairs of data where a relationship is suspected.
  Draw a graph with the independent variable on the horizontal axis and the dependent variable on the vertical axis. For each pair of data, put a dot or a symbol where the x-axis value intersects the y-axis value. (If two dots fall together, put them side by side, touching, so that you can see both.)
  Look at the pattern of points to see if a relationship is obvious. If the data clearly form a line or a curve, you may stop. The variables are correlated. You may wish to use regression or correlation analysis now.  

Example:

Variable A = the number of employees trained on new software.
Variable B = the number of bugs identified by users or customers post release into production.
Plot the above for various projects and see there exists a correlation between these 2 variables.You suspect that more training reduces the number of bugs. 

Even if the scatter diagram shows a relationship, do not assume that one variable caused the other. Both may be influenced by a third variable.

When the data are plotted, the more the diagram resembles a straight line, the stronger the relationship.

If a line is not clear, statistics determine whether there is reasonable certainty that a relationship exists. If the statistics say that no relationship exists, the pattern could have occurred by random chance.

Think creatively about how to use scatter diagrams to discover a root cause.

Drawing a scatter diagram is the first step in looking for a relationship between variables.