A First Course in Quality Engineering, 2nd Edition

Statistics for Quality 89

Although a random variable may seem to behave in a haphazard or cha-

otic manner to an ordinary observer, statisticians have found that they fol-

low certain patterns and that the behavior can be captured in models� These

models, represented by formulas, graphs, or tables, are called “probability

distributions�” Because the two types of random variables mentioned above

are mathematically different, one being continuous and the other noncon-

tinuous, they need two different types of models� The model used to describe

a discrete random variable is called the “probability mass function,” and that

used to describe a continuous random variable is called the “probability den-

sity function�” First, we will discuss the probability ...