Tool 114: Multivariable Chart

AKA

Multi-Var Chart

Classification

Analyzing/Trending (AT)

Tool description

A multivariable chart is used to measure time-series data of multiple variables reflecting process capability variance. This chart provides process variable correlation and interaction information that is not usually found when examining traditional control charts one at a time.

Typical application

• To construct an overlay of certain process variables normally recorded on control charts.

• To allow time-series analysis of process variables.

• To identify possible problem causes.

• To contribute to design of experiments (DOE) and statistical process control (SPC) activities.

Problem-solving phase

→	Select and define problem or opportunity
→	Identify and analyze causes or potential change
	Develop and plan possible solutions or change
	Implement and evaluate solution or change
	Measure and report solution or change results
	Recognize and reward team efforts

Typically used by

1	Research/statistics
	Creativity/innovation
3	Engineering
	Project management
2	Manufacturing
	Marketing/sales
	Administration/documentation
	Servicing/support
4	Customer/quality metrics
	Change management

links to other tools

befor

• Control Chart

• Data Collection Strategy

• Checksheet

• Checklist

• Standard Deviation

after

• Variance Analysis

• Process Capability Ratios

• Analysis of Variance

• Potential Problem Analysis (PPA)

• Trend Analysis

Notes and key points

• Note that it is difficult plotting process variables along matching time spans. Also, scaling of upper and lower specification limits (USL-LSL) for process variables may be limited to the base variable with the greatest upper and lower deviation from the specification target value.

Step-by-step procedure

• STEP 1 First, acquire the target and upper and lower specifiction values from design engineering, manufacturing, or the quality department.

• STEP 2 Identify two to four related process variables. See example Painting Quality.

• STEP 3 Draw a chart, with the center line labeled spec (target) value and upper and lower horizontal lines designated USL and LSL respectively.

• STEP 4 Designate the x-axis with the proper time scale. The x-axis represents an amount of time for the variable with the longest time span.

• STEP 5 Identify process variables and encode for plotting and analysis purposes.

• STEP 6 Take measurements and plot by connecting data points.

• STEP 7 Date the chart and keep for later reference.

Example of tool application