Chapter 3
Exploratory Data Analysis, Display and Summary of Multivariate Data
Display is an obligation! (Tukey, 1986)
Overview
Two tools of exploratory data analysis (EDA), the stem-and-leaf plot and boxplot, provide versatile and informative displays of process data and enable outliers to be defined and detected. The brushing facility in Minitab is introduced as it enables subsets of data sets displayed in graphs to be readily identified and explored.
There are many situations where two or more performance measurements are made in assessing process performance, so some familiarity with techniques for the display and summary of bivariate and multivariate data is important.
3.1 Exploratory Data Analysis
3.1.1 Stem-and-Leaf Displays
In 1998 when the author worked in Dunfermline, Scotland, he was involved in a process that many of us undertake every day – the process of driving to work. The route from Loanhead, across the Forth Road Bridge, was 25 miles long. A run chart of journey duration (minutes) for 32 journeys undertaken during September and October 1998 is shown in Figure 3.1.
Figure 3.1 Run chart of journey duration.
The low P-value for clustering indicates the possible influence on journey duration of some special causes. In addition to journey duration, the weather was recorded as either dry (D) or wet (W). The author had a hunch that this (uncontrollable!) factor influenced ...
