2.
Measurement Process Characterization
2.1. Characterization 2.1.2. What is a check standard?


Case study: 
Before applying the quality control procedures recommended
in this chapter to check standard data, basic assumptions
should be examined. The basic assumptions underlying the
quality control procedures are:


An easy method for checking the assumption of a single
normal distribution is to construct a histogram of the check
standard data. The
histogram
should follow a
bellshaped pattern with a single hump. Types of anomalies that indicate
a problem with the measurement system are:


Another graphical method for testing the normality assumption is a probability plot. The points are expected to fall approximately on a straight line if the data come from a normal distribution. Outliers, or data from other distributions, will produce an Sshaped curve.  
A graphical method for testing for correlation among measurements is a timelag plot. Correlation will frequently not be a problem if measurements are properly structured over time. Correlation problems generally occur when measurements are taken so close together in time that the instrument cannot properly recover from one measurement to the next. Correlations over time are usually present but are often negligible. 