Variable Control Charts

Minitab Reference

A variable control chart plots continuous data measures over time, basically there are two main types of this kind of control chart: individual measurements charts and charts for data collected in subgroups.

Variables Control Charts for Subgroup Data


Each point on the graph represents a subgroup; that is, a group of units produced under the same set of conditions. For example, you want to chart a particular measurement from your process. If you collect and measure five parts every hour, your subgroup size would be 5.

Variables control charts for subgroups include Xbar, R, S, and Zone.

control charts for subgroups

Xbar chart - Plots the process mean over time. Use to track the process level and detect the presence of special causes affecting the mean.

R chart - Plots the process range over time. Use to track process variation and detect unexpected variation.

S chart - Plots the process standard deviation over time. Use to track the process variation and detect unexpected variation.

Zone chart - Plots the cumulative scores based on "zones" at 1, 2, and 3 standard deviations from the center line. Use to detect unexpected variation.

 

Variables Control Charts for Individuals Data


Each point on the graph represents an individual measurement; thus, the subgroup size is 1. Individuals charts are used when measurements are expensive, production volume is low, or products have a long cycle time; for example, to test the impact strength of parts (destructive testing). Individuals control charts include I charts and MR charts.

individual charts

I chart - Plots individual observations over time. Use to track the process level and detect the presence of special causes.

Moving range chart - Plots the moving range over time. Use to track the process variation and detect the presence of special causes.