Implications of CX Consistency for Researchers – Part 3 – Common Cause v Special Cause Variation

Previously, we discussed the implications of intra-channel consistency for researchers.

This post considers two types of variation in the customer experience: common and special cause variation, and their implications for customer researchers.

The concepts of common and special cause variation are derived from the process management discipline Six Sigma.

Common cause variation is normal or random variation within the system.  It is statistical noise within the system.   Examples of common cause variation in the customer experience are:

• Poorly defined, poorly designed, inappropriate policies or procedures
• Poor design or maintenance of computer systems
• Inappropriate hiring practices
• Insufficient training
• Measurement error

Special cause variation, on the other hand, is not random.  It conforms to laws of probability.   It is the signal within the system.  Examples of special cause variation include:

• High demand/ high traffic
• Poor adjustment of equipment
• Just having a bad day

What are the implications of common and special cause variation for customer experience researchers?

Given the differences between common cause and special cause variation, researchers need a tool to help them distinguish between the two.  Researchers need a means of determining if any observed variation in the customer experience is statistical noise or a signal within the system.  Control charts are a statistical tool to make a determination if variation is noise or a signal.

Control charts track measurements within upper and lower quality control limits.  These quality control limits define statistically significant variation overtime (typically at a 95% confidence), which means there is a 95% probability that the variation is the result of an actual change in the customer experience (special cause variation) not just normal common cause variation.  Observed variation within these quality control limits are common cause variation.  Variation which migrates outside these quality control limits is special cause variation.

To illustrate this concept, consider the following example of mystery shop results:

This chart depicts a set of mystery shop scores which both vary from month to month and generally appear to trend upward.

Customer experience researchers need to provide managers a means of determining if the month to month variation is statistical noise or some meaningful signal within the system.  Turning this chart into a control chart by adding statistically defined upper and lower quality control limits will determine if the monthly variation is common or special cause.

To define quality control limits, the customer experience researcher needs to determine the count of observations for each month, the monthly standard deviation, and the average count of shops across all months.

The following table adds these three additional pieces of information into our example:

To define the upper and lower quality control limits (UCL and LCL, respectively), apply the following formula:

Where:

x = Grand Mean of the score

n = Mean sample size (number of shops)

SD = Mean standard deviation

These equations yield quality control limits at 95% confidence, which means there is a 95% probability any variation observed outside these limits is special cause variation, rather than normal common cause variation within the system

Calculating these quality control limits and applying them to the above chart produces the following control chart, with upper and lower quality control limits depicted in red:

This control chart now answers the question, what variation is common cause and what variation is special cause.  The general trend upward appears to be statistically significant with the most recent month above the upper quality control limit.  Additionally, this control chart identifies a period of special cause variation in July.  With 95% confidence we know some special cause drove the scores below the lower control limit.  Perhaps this special cause was employee turnover, perhaps a new system rollout, or perhaps a weather event that impacted the customer experience.