We start talking about probability distribution in chapter 2.2.3 Normal Distributions & Normality. So you already know that:


  • When you don’t have all the process data, you need to work on a representative sample;
  • Plotting this sample with descriptive statistics (like a graph), help you to find the correct type of distributions;
  • With the correct type of distribution, you can assess the probability of an event in a process.


Finally, knowing the probability of an event in a process can help you to reply to questions like:

  • How many non-conforming units can I have in a process?
  • What’s the number of non-conformity can I have in a process? (maybe a unit with more than one error)
  • How many failures can I have in a trial before the first success?


In this case, you are basing your response on only a tiny sample instead of counting the entire population, which in the same case, it’s impossible or too expensive.

The main kind of distribution are:

  1. Discrete distribution: when you have a variable that assumes only a countable value. For example, the face of a coin are two and are countable;
  2. Continuos distribution: when you have a variable that assumes a non-countable value. So the value that it can be assumed is infinite in a range. For example, the temperature in a room can be considered to take an endless number of values.
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