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When an error occurs in a process, it’s often tempting to insert an inspection step in the process. Nuclear power plant procedures are full of inspection steps, often due to “one-time” occurrences.

The principles of error-proofing, though, provide guidelines that result in process changes that prevent or mitigate the error without adding an inspection step. These principles were derived from examining many hundreds of human errors and the actions taken by organizations to prevent their reoccurrence.

Whenever possible, adopting one of the "error prevention" strategies is preferred, since the error is prevented from happening in the first place.

The "error confinement" strategies are more "downstream" in nature, and they try to detect the error before it has a chance to result in an undesirable effect.

Error Prevention

A. Error Elimination - Take action to eliminate the possibility of the error occurring. Examples: 1) To prevent burns, instead of a "Do Not Touch" sign; insulate pipes carrying high temperature fluids. 2) Provide switches to keep an operator’s hands away from dangerous stamping machines.

B. Decision Delegation - Take action to eliminate the need to make a routine decision. Examples: 1) Provide a torque wrench with an audible "click" when the desired torque is reached, instead of requiring an operator to read a torque value. 2) Automate the cash register to subtract the amount received from the customer from the bill to prevent change errors.

C. Task Facilitation

1. Match the task to the operator's abilities - Examples: 1) Attach carrying handles to bulky objects. 2) Provide easy-to-read instructions with graphics for assembly, or provide a video tape of assembly instructions.

2. Task stratification and specialization - Example: Separate similar tasks spatially (performed in different locations) or time-wise (Task "A" this week, Task "B" next week); place similar parts in different bins.

3. Task Distinguishment - Example: Color coding of similar parts (e.g. wiring, drug containers); different color identification tags for equipment on adjacent power plants.

Error Confinement

A. Error Detection Methods - Design means of detecting errors (as close to the source of the error as possible):

1. Methods based on physical movement - Example: An assembly process requires 10 drillings. If the proper number of holes is not drilled, an automatic alarm will sound.

2. Methods based on detecting errors post-installation - Example: Use different bolt patterns for assembly work (e.g. exhaust manifold to engine block).

B. Error Mitigation Methods - Employ methods that mitigate the effects of the error - Examples: 1) If driver error causes the automobile to crash, safety belts and airbags mitigate the effects of the error. 2) If gloves are required to be worn near rotating machinery, make sure they “tear-away” if the operator’s fingers are caught in the machine.

4. TOOLS FOR IMPROVEMENT

This section will introduce you to three tools that are very, very helpful in process improvement work. Although they are simple tools to use, they are very powerful. We’ve got a lot of statistical tools in our quality tool chest, but these seem to be the ones that are used the most. Here’s the kinds of questions they’ll help you answer:

Question Analysis Type The Picture

What’s most important?

Example: A Pareto Analysis of Locomotive Failures showed Low Voltage problems to be most frequent.

Pareto

Categories Frequency

%

How do the values of a data set vary, what is the shape of the data?

Example: A Histogram of Axle Inner Sleeve Diameters showed possible out-of-spec conditions.

Variation/ Histogram

Value Frequency

Are two variables related to each other, perhaps through a cause and effect relationship?

Example: The reaction rate of the chemical process is positively related to the temperature of the process.

Correlation/ Scatter Diagram

Independent Variable Dependent

Variable

4.1 PARETO ANALYSIS The Pareto Principle

Back in the late 1800’s, an Italian economist, Vilfredo Frederico Pareto, came up with an interesting finding: About 80% of the wealth of his country was held by fewer than 20% of the people. Let’s fast forward to the 1930’s. In studying manufacturing defects, Dr. Joseph Juran observed that often over 80% of the problems were caused by only a few factors or variables in the production process.

His familiarity with the work of V. F. Pareto led him to name this empirical observation The Pareto Principle.¹⁴ The Pareto Principle has broad application in quality improvement. A few examples of the Pareto Principle at work:

• Most of a hospital’s admissions are due to a relatively few physicians,

• A large fraction of power plant failures are due to boiler tube problems,

• Seventy percent of assembly defects on irrigation sprinklers are due to problems inserting only two components,

• Over 75% of a printing company’s sales are to just three customers.

• Wasted days in a hospital are due mainly to problems transferring patients to Skilled Nursing Facilities,

• Medical Record errors occur most often in three parts of the form,

• Delays receiving spare parts are most often associated with one vendor,

• Over 90% of Mutual Fund transaction errors fall into four categories,

• Sixty-five percent of employee injuries are back strains and sprains,

• Over 70% of airline accidents are due to human error.

Although most of the examples apply to problems encountered in production processes, Pareto can also apply to sales, volumes, costs and other quality characteristics of a product or service.

14Dr. Juran humorously laments that he didn’t name it after himself. He says that he was there, he had the opportunity, but he blew it!

Now we won’t guarantee that the Pareto Principle will appear in every situation, but it does pretty often. In fact, it appears often enough that we’ve found it worthwhile to include an attempt at Pareto Analysis in almost every process improvement effort in which we’re involved.

There’s even a sort of Pareto Principle that’s applied to quality tools: With just Pareto and Cause & Effect (see Section 5), over 80% of quality problems can be addressed.