The 3Ts error proofing method is universal applicable everywhere human beings work and make decisions throughout an organisation

By using 3T ‘Target – Tolerance – Test’ mistake proofing on a job task you create a parallel proof test activity to check that the task was done right. Once you add a proof test to a task the error rate for the combined activity can fall 10,000%.



Hello Mike,

I have just skimmed Mike Sondalini’s book on Employee Training and Development with Standard Operating Procedures. Are these methods applicable to software development? Do you have any experience applying these methods to non-routine business processes such as engineering (in particular software engineering)?



Dear John,

The methods in the Employee Training and Development with Standard Operating Procedures book are universally applicable in every human activity. The human error prevention techniques in the book are used to give people mastery and control over what they do. If you use people in delivering a result then every tool and method shown in the Employee Training and Development with Standard Operating Procedures book will help your people do great work.

All human performance is probabilistic; the results of our work, the outcomes of our choices and the winners in our games depend on many things happening. For every activity, action and decision there is a range of possible outcomes, from terrible to magnificent and all results in-between. The 3Ts add control into a situation so that the intended result is delivered and the outcome is far less dependent on chance and luck. When developing a 3T SOP we reduce the influence of chance by setting the range of results we want and testing that the right result is achieved before going onto the next step.

Every job is a series of activities. Every software program is a series of coded instructions. As soon as a series arrangement exists you have a ‘chain’ that can break at every link. One human error anywhere in a series of tasks and the whole job is failed.

Putting human error preventing 3Ts into job procedures makes you apply the Carpenter’s Creed—measure twice, cut once—when doing a task. The second measurement is a proof test that the first result was right. The combined probability of two tests both being done wrong makes the chance of passing a defect or fault into service rare. To get both measurements wrong one after the other is very unlikely (it is possible that the first test and the proof test are both wrong, but such probability is low in a controlled situation). There is one proviso—the measuring system used (i.e. the measuring equipment and the people doing the measuring) must be accurate and independent of each other. There must be no collusion between the two tests to arrive at an agreed result.

If you are ever diagnosed with a serious illness always get a competent second opinion that is totally independent of the first one. Doctors are notorious for the high error rate of their diagnoses. In fact, a third totally independent analysis and opinion is necessary at times so that you can be truly confident of the real situation.

The 3T SOP layout and the use of flow chart procedures are visual techniques that help people understand what is required of them and what ‘right’ looks like. Unless you know exactly what is ‘right’ and you know the exact way to get that result you are at the mercy of hope and luck that no one makes a mistake. The methods in Employee Training and Development with Standard Operating Procedures let you introduce controls, measurement and checking into human activities so that people know what results they are producing. If their outcome is not yet good enough they use the learning from applying the 3Ts to adjust and correct what they do until it at least meets the worst allowable performance.

When you first decide to use the methods in the Employee Training and Development with Standard Operating Procedures book you start an experiment. The 3T accuracy control principles are sound but you do not yet know that, and you are not confident that they will work. To use the 3Ts the first time means you are taking a risk. All risks that have negative impact should be entered into under controlled and managed conditions. First gain experience and confidence in writing 3T SOPs and in using the in-built Carpenter’s Creed controls. The advice when using the 3T accuracy controlled method the first few times is to test it against your current practise.

3T ‘Target – Tolerance – Test’ mistake proofing is a universal principle to help human beings do top quality work. Software development is totally dependent on human beings. The effects of software errors can be catastrophic and long lasting. If you see a place where you can incorporate 3T mistake proofing during software development then test it. Imagine what it means if you can reduce the error rate by 10,000%.

Let me know if you need more information.


My best regards to you,

Mike Sondalini
Managing Director
Lifetime Reliability Solutions HQ


PS. If you require advice on ISO 9001 quality system development, please feel free to contact me by email at