In Chapters 5-8 we have considered Shewhart control charts for variables and attributes, named after the man who first described them in the 1920s. The basic rules for the operation of these charts predominantly concern the interpretation of each sample plot. Investigative and possibly corrective action is taken if an individual sample point falls outside the action lines, or if two consecutive plots appear in the warning zone – between warning and action lines. A repeat sample is usually taken immediately after a point is plotted in the warning zone. Guidelines have been set down in Chapter 6 for the detection of trends and runs above and below the average value but, essentially, process control by Shewhart charts considers each point as it is plotted. There are alternative control charts which consider more than one sample result.