Chapters 6.1 & 6.2
The control chart is a graph used to study how a process changes over time. Data are plotted in time order. A control chart always has a central line for the average, an upper line for the upper control limit and a lower line for the lower control limit. These lines are determined from historical data.
Out of Control Signals
We call the system out of control if at least one of the three events occur
- Out of Control Signal 1: At least one point falls beyond the 3s level.
- Out of Control Signal 2: A run of nine consecutive points is on the same side of the center line (usually the mean).
- Out of Control Signal 3: At least two of three consecutive points lie beyond the 2s level on same side of the center line (usually the mean).
The standard score (more commonly referred to as a z-score) is a very useful statistic because it (a) allows us to calculate the probability of a score occurring within our normal distribution and (b) enables us to compare two scores that are from different normal distributions.