Mistake proofing Mistake proofing is a scientific technique for improvement of operating systems including materials, machines and methods with an aim of preventing problems due to human error. The term “error” means a sporadic deviation from standard procedures resulting from loss of memory, perception or motion.

Defect Vs errors It is important to understand that defects and errors are not the same thing. A defect is the result of an error, or an error is the cause of defects as explained below. Cause Result Error Defect

Prevention of defects End result Cause Intermediate result zero defect Work Procedure Machine or human error Detect error Take corrective action Modify work procedure to prevent such errors Analyze for preventive action

Types of error Error in memory of PLAN : Error of forgetting the sequence/ contents operations required or restricted in standard procedures. Error in memory of EXECUTION : Errors of forgetting the sequence/contents of operations having been finished. Error in PERCEPTION of type : Error of selecting the wrong object in type or quantity.

Types of error Error in perception of MOVEMENT : Error of misunderstanding/misjudging the shape, position, direction or other characteristics of the objects. Error in motion of HOLDING : Error in failing to hold objects. Error in motion of CHANGING : Errors of failing to change the shape , position , direction , or other characteristics of object according to the standard.

Human error provoking situations Complex design Inadequate written standards Too many parts Mix up Too many steps Specifications or critical conditions Too many adjustments Frequent repetition

Principles of mistake proofing The principles of mistake proofing can be categorized into groups: A. Prevention of occurrence B. Minimization of effects

A. Prevention of occurrence Methods under this principle aim to prevent the occurrence of human errors from all stages of operations and make corrections unnecessary.This can be done through the following 3 methods:- I. Elimination II. Replacement III. Facilitation

I. Elimination Elimination method aims to remove the system properties which generate operations/restrictions susceptible to human errors so as to make them unnecessary. Consider the error of an operator touching a high temperature pipe and getting burnt. One method of preventing this error is to make the pipe safe by covering it with insulating material. This improvement removes the property of the pipe which generates the restriction.

III. Facilitation The principles of elimination and replacement described above make it unnecessary for operators to perform operations or note restrictions. In contrast to these, the purpose of facilitation is to make such functions as memory, perception and motion required in operations easy for the operators by well designed work stations and thus reduce the occurrence rate of human errors.

B. Minimization of effects* Methods under this principle aim to minimize the effects of human error and focus on processes where the errors develop into serious problems of quality,safety or efficiency. This can be divided into two categories : I. Detection and II. Mitigation *This principle is also known as Stop-in-time.

I. Detection The concept of detection is that even if a human error occurs, the deviations from standard states caused by it can be detected and corrected in the succeeding operation steps. The methods of correcting the detected deviations are classified into two types : 1. Operators are informed or find by themselves the deviations and take necessary corrective action. 2. The deviations are automatically corrected without operators

Detection There are two methods of informing the operator. The first method is to bring errors to the operator’s notice by visual or audible alarms, and the second method is by making the successive operations impossible.

II. Mitigation The mitigation method aims to make operations redundant or incorporate shock absorbing/protecting materials so as to mitigate the effects of human errors in their development process. Example : Consider the error of burning out the motor of a production machine by forgetting to switch off the power supply. The mitigation countermeasure is to install a fuse which cuts off the power supply when the temperature reaches certain point.