Thoughts on Variation

Don’t attribute (good or bad) to people that which should be attributed to the system.

What is variation?

  • a change or difference in condition, amount, or level, typically with certain limits

Impact of Variation

Variation results in:

  • adverse patient safety events
  • reduced job satisfaction
  • loss of productivity
  • loss of revenue
  • …so we should try and reduce variation.

Types of Variation

There are two types of variation and they require different approaches to remove them

Common-cause variation is characterised by:

  • Phenomena constantly active within the system;
  • Variation predictable probabilistically;
  • Irregular variation within an historical experience base; and
  • Lack of significance in individual high or low values.

Special-cause variation is characterised by:

  • New, unanticipated, emergent or previously neglected phenomena within the system;
  • Variation inherently unpredictable, even probabilistically;
  • Variation outside the historical experience base; and
  • Evidence of some inherent change in the system or our knowledge of it.

Determining Variation Types

Hey Craig! How can we understand what kind of variation we are dealing with?

Common Cause Examples

  • Lack of clearly defined standard operating procedures
  • Inappropriate procedures
  • Normal wear and tear
  • Variability in settings
  • Computer response time
  • Poor design
  • Poor maintenance of machines
  • Poor working conditions, e.g. lighting, noise, dirt, temperature, ventilation
  • Substandard raw materials
  • Ambient temperature and humidity
A flat tire is common cause variation
A flat tire is common cause variation. It’s normal wear and tear. Which is why all vehicles are equipped with a spare tire, jack etc.

Special Cause

  • Operator falls asleep
  • Machine malfunction
  • Computer crashes
  • Poor batch of raw material
  • Power surges
  • Broken part
  • Abnormal traffic (click-fraud) on web ads
  • Extremely long lab testing turnover time due to switching to new computer system
  • Operator absent
4 slashed tires is special cause variation
Someone slashing all four of your tires is special cause variation. It’s not anticipated and there is not much you can do about it.

Responding to Variation

  • Response to common cause variation is incremental and requires data (measurement)
  • Response to special cause tends to be knee-jerk and expensive.

So why do we care Craig?

We should care because more often than not the wrong type of variation is suspected and then the wrong response is applied (tampering), which is not only more expensive but it negatively impacts the staff working within the system.

For example, what was the reaction to the 9/11 attacks in the United States? They spent billions of dollars tampering with the system (a common cause response) that ultimately has not increased safety because it was not the source of the variation in the first place. The nature of special cause variation is that it never happens the same way twice (which is why the terrorists opted for shoe explosives instead of box cutters the next time around).

For more information I highly recommend this post at the Gemba Walkabout: