Rules of 3 and 5:

It has been a while since I have blogged about statistics. So in today’s post, I will be looking at rules of 3 and 5. These are heuristics or rules of thumb that can help us out. They are associated with sample sizes.

Rule of 3:

Let’s assume that you are looking at a binomial event (pass or fail). You took 30 samples and tested them to see how many passes or failures you get. The results yielded no failures. Then, based on the rule of 3, you can state that at 95% confidence level, the upper bound for a failure is 3/30 = 10% or the reliability is at least 90%. The rule is written as;

p = 3/n

Where p is the upper bound of failure, and n is the sample size.

Thus, if you used 300 samples, then you could state with 95% confidence that the process is at least 99% reliable based on p = 3/300 = 1%. Another way to express this is to say that with 95% confidence fewer than 1 in 100 units will fail under the same conditions.

This rule can be derived from using binomial distribution. The 95% confidence comes from the alpha value of 0.05. The calculated value from the rule of three formula gets more accurate with a sample size of 20 or more.

Rule of 5:

I came across the rule of 5 from Douglas Hubbard’s informative book “How to Measure Anything” ^[1]. Hubbard states the Rule of 5 as;

There is a 93.75% chance that the median of a population is between the smallest and largest values in any random sample of five from that population.

This is a really neat heuristic because you can actually tell a lot from a sample size of 5! The median is the 50^th percentile value of a population, the point where half of the population is above it and half of the population is below it. Hubbard points out the probability of picking a value above or below the median is 50% – the same as a coin toss. Thus, we can calculate that the probability of getting 5 heads in a row is 0.5^⁵ or 3.125%. This would be the same for getting 5 tails in a row. Then the probability of not getting all heads or all tails is (100 – (3.125+3.125)) or 93.75%. Thus, we can state that the chance of one value out of five being above the median and at least one value below the median is 93.75%.

Final words:

The reader has to keep in mind that both of the rules require the use of randomly selected samples. The Rule of 3 is a version of Bayes’ Success Run Theorem and Wilk’s One-sided Tolerance calculation. I invite the reader to check out my posts that sheds more light on this 1) Relationship between AQL/RQL and Reliability/Confidence , 2) Reliability/Confidence Level Calculator (with c = 0, 1….., n) and 3) Wilk’s One-sided Tolerance Spreadsheet.

When we are utilizing random samples to represent a population, we are calculating a statistic – a representation value of the parameter value. A statistic is an estimate of the parameter, the true value from a population. The higher the sample size used, the better the statistic can represent the parameter and better your estimation.

I will finish with a story based on chance and probability;

It was the finals and an undergraduate psychology major was totally hung over from the previous night. He was somewhat relieved to find that the exam was a true/false test. He had taken a basic stat course and did remember his professor once performing a coin flipping experiment. On a moment of clarity, he decided to flip a coin he had in his pocket to get the answers for each questions. The psychology professor watched the student the entire two hours as he was flipping the coin…writing the answer…flipping the coin….writing the answer, on and on. At the end of the two hours, everyone else had left the room except for this one student. The professor walks up to his desk and angrily interrupts the student, saying: “Listen, it is obvious that you did not study for this exam since you didn’t even open the question booklet. If you are just flipping a coin for your answer, why is it taking you so long?”

The stunned student looks up at the professor and replies bitterly (as he is still flipping the coin): “Shhh! I am checking my answers!”

Always keep on learning…

In case you missed it, my last post was Kenjutsu, Ohno and Polanyi:

[1] How to Measure Anything.

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