Scatter Diagram

Also Known As: 
Scatter Plot
X-Y Graph
Description: 

A scatter diagram consists of pairs of numerical data containing one variable on each axis. The diagram is used to find a relationship between the data pairs. Points that create a line or curve indicate correlated variables.

Uses: 

To show what happens to a variable when another is changed.

When analyzing whether two variables are related.

When working with paired, quantitative data.

How do I use this tool?: 

1. DEFINE THE X VARIABLE, typically the "cause variable," on a graph paper scatter diagram form.

2. DEFINE THE Y VARIABLE, typically the "effect variable," on the diagram.

3. NUMBER THE PAIRS OF X AND Y VARIABLE MEASUREMENTS CONSECUTIVELY. Put each data pair for x and y in their respective columns. Ensure the data remains paired.

4. PLOT THE X AND Y DATA PAIRS on the diagram. Place a point on the graph where the two values intersect.

5. STUDY THE SHAPE that is created by the series of data points plotted. If there is a data association the shape should resemble an elliptical shape or a straight line.
a. There is little or no correlation if the points form a circular shape.
b. The correlation is positive if the points for both values are increasing.
c. The correlation is negative if one variable increases and another decreases.

Expertise Required: 
Users of this tool usually have some training or experience in its use.
Recommended Supplies/Materials: 
Paired data
Spreadsheet software
Advantages: 

Spreadsheet software makes this method easy.

Reveals patterns in data.

Is an easy way of determining whether a cause-and-effect relationship between two variables exists.

Provides the first step towards analyzing the relationship between two variables.

Disadvantages: 

If the data cover too small of a range the results of the analysis can be skewed.

Patterns can appear or vanish based on how the data are stratified.

If there is a pattern, it does not always indicate that there is a relationship.

Where can I go to learn more?: 

University Research Co. LLC. Health care improvement project: scatter diagram. 2008 [cited 2009 July 28]; Available from: http://www.hciproject.org/improvement_tools/improvement_methods/analytical_tools/statistical_data_presentation/scatter_diagram

George M, Rowlands D, Price M, et al. Identifying and verifying causes. The lean six sigma pocket toolbook. New York: McGraw - Hill; 2005. p. 141-96.

American Society for Quality. Cause analysis tools: scatter diagram. 2009 [cited 2009 July 23]; Available from: http://www.asq.org/learn-about-quality/cause-analysis-tools/overview/scatter.html

Last Modified: May 2013