We will be studying the following techniques:

- The stem-and-leaf plot
- Letter values
- Data re-expression
- Bagplot—a bivariate boxplot
- Resistant line
- Smoothing data
- Median polish

The stem-and-leaf plot is considered as one of the seven important tools of **Statistical Process Control** (**SPC**); refer to Montgomery (2005). It is a bit similar in nature to the histogram plot.

The stem-and-leaf plot is an effective method of displaying data in a (partial) tree form. Here, each datum is split into two parts: the stem part and the leaf part. In general, the last digit of a datum forms the leaf part; the rest form the stem. Now, consider a datum *235*. If the split criteria is the units place, the stem and leaf parts ...

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