Scatter Diagram

Scatter Diagrams (or Plots) are used to visually represent data for further analysis in correlation or regressionThe diagram shows a pair of numerical data, one for each axis (horizontal and vertical with are x-y respectively). 

They explore associations between two variables and the "x" variable (input) is varied systematically and the "y" (output) is measured once the input changes. The intention is to visually examine trends or patterns of the input (x) onto the response in the experiment. 

The chart below would indicate a weak negative linear correlation. From here the Coefficient of Correlation can be determined as well as best fit line to describe the behavior of the data. 

Negative correlation on a Scatter PlotA possible negative correlation but not overly convincing

There are no assumptions of normality or sequence to use a Scatter Plot. Sequential plotting is required for a Time-Series plot (and for SPC). In other words, the data for Scatter Plots does not have to be in sequence or chronological order but this is required for SPC and Time-Series charts. 

It is most useful when there is a lot of data in table and very difficult to tell if there is any type, and to what degree, of correlation. 

As association of variables does not always imply cause. There could be other lurking variables affecting the measured "y" response.

A scatter plot of the blue dots shown below makes it very obvious that there is strong positive linear correlation but it does not provide any value other than a visual indication. 

Scatter Plot, Scatter DiagramSix-Sigma-Material.com

A Scatter diagram itself is limited in its use but is rather a starting point for further analysis of correlation and regression.

This is another one of the many visual tools (such as a Box Plot and Pareto Chart)  that a Six Sigma project manager can use to help break down data into more meaningful terms.

Line charts are similar to Scatter Plots but contain lines connecting the data.

Scatter plots can make it easier to find a correlation as most of it is not easy to see a non-linear correlation amongst thousands of lines of data.

The chart above is an example of the value of the plot. Simply looking at a set of x-y data on a sheet would be a challenge to see this relationship. There appears to be a strong non-linear relationship with a few outliers within the range of 0-200 units.

Those outliers are concerning. There are only a few of them so it would be worthwhile understanding what happened in each case. 

7 Basic Quality Tools

A Scatter Plot is considered 1 of 7 Basic Quality Tools for process improvement. The others are:

If you're considering a Six Sigma Certification exam or a PMP certification, memorize and understand the purpose of these 7 tools.





Return to BASIC STATISTICS

Return to the ANALYZE Phase

Templates, Tables, and Calculators

Practice certification problems

Six Sigma Jobs


Recent Articles

  1. Process Capability Indices

    Oct 18, 21 09:32 AM

    Determing the process capability indices, Pp, Ppk, Cp, Cpk, Cpm

    Read More

  2. Six Sigma Calculator, Statistics Tables, and Six Sigma Templates

    Sep 14, 21 09:19 AM

    Six Sigma Calculators, Statistics Tables, and Six Sigma Templates to make your job easier as a Six Sigma Project Manager

    Read More

  3. Six Sigma Templates, Statistics Tables, and Six Sigma Calculators

    Aug 16, 21 01:25 PM

    Six Sigma Templates, Tables, and Calculators. MTBF, MTTR, A3, EOQ, 5S, 5 WHY, DPMO, FMEA, SIPOC, RTY, DMAIC Contract, OEE, Value Stream Map, Pugh Matrix

    Read More

Custom Search


Site Membership
LEARN MORE


Six Sigma

Templates, Tables & Calculators


Six Sigma Slides

CLICK HERE

Green Belt Program (1,000+ Slides)

Basic Statistics

Cost of Quality

SPC

Process Mapping

Capability Studies

MSA

SIPOC

Cause & Effect Matrix

FMEA

Multivariate Analysis

Central Limit Theorem

Confidence Intervals

Hypothesis Testing

T Tests

1-Way ANOVA

Chi-Square

Correlation

Regression

Control Plan

Kaizen

MTBF and MTTR

Project Pitfalls

Error Proofing

Z Scores

OEE

Takt Time

Line Balancing

Yield Metrics

Sampling Methods

Data Classification

Practice Exam

... and more



Statistics in Excel


Need a Gantt Chart?

Click here to get this template