In chapter 2.2.1/2 Basic and Descriptive Statistics, we look at descriptive statistics used to organize, summarize, and present data. In this chapter, we want to focus on the inferential statistics used to make a conclusion on a population starting from data in a sample.

Tools of the statistical inference that we will use in six sigma are Hypothesis testing, which can test if a particular sample or outcome is representative for all the population where the sample was initially drawn. In addition, you use the probability to look at how confident you are that the conclusion is correct.

Remember that:

  • A statistics is a value obtained from the sample;
  • A parameter is a value found or estimated from the population.

So we can say that you have a population where you don’t know the parameter, maybe because it is too large. So you select a sample from the population, then calculate the sample statistics to estimate the population parameter.

Example: We have the hypothesis that the new offer of a company is made in an average of 28 days, but we don't have the data of all the projects (the population), so we can't estimate the parameter (simply calculate the mean). For this reason, we used a sample, and we made statistics on it.
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