Statistics for Process Control Engineers

6Presenting the Data

Many of the ways that data are analysed for presentation do nothing to help the engineer design and support control schemes. A pie chart, for example, cannot readily be used to estimate the benefits captured by improved control or to assess the accuracy of an inferential property. But one of the key responsibilities of a process control engineer is to present data to others that have little understanding of the technology. This might be to senior management in order to obtain approval of major expenditure on an advanced control project. It might be to the maintenance department to highlight how poor performing instrumentation is restricting the benefits being captured by improved control. It might be to the laboratory drawing attention to unexplained differences between their results and an inferential property or an on‐stream analyser. With a little imagination, even a simple pie chart can be used to great effect to illustrate a key issue to others.

This chapter covers techniques that might be used for such presentation, along with those that can also be used for engineering.

6.1 Box and Whisker Diagram

Figure 6.1 is box and whisker diagram drawn for the 100 results for C₂ in propane cargoes, shown as Table A1.2. The width of the box is the interquartile range – in this case 0.925. The division is the median – in this case 3.5. In this form of the diagram, the ends of whiskers represent the range of data – 1.9 to 5.0. There are other forms in which the ...