DESCRIPTIVE STATISTICS
Descriptive statistics can be helpful in describing certain characteristics of a product and a process. The most important descriptive statistics are measures of central tendency such as the mean, measures of variability such as the standard deviation and range, and measures of the distribution of data. We first review these descriptive statistics and then see how we use them to measure changes in product and process characteristics.
Mean (average)
A statistic that measures the central tendency of a set of data.
The Mean
In the soft-drink bottling example, we stated that the average bottle is filled with 16 ounces of liquid. The arithmetic average, or the mean, is a statistic that measures the central tendency of a set of data. Knowing the central point of a set of data is highly important. Just think how important that number is when you receive test scores!
To compute the mean, we simply sum all the observations and divide by the total number of observations. The equation for computing the mean is
where 
= the mean
xi = observation i, i = 1, . . ., n
n = number of observations
The Range and Standard Deviation
In the bottling example, we also stated that the ...
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