In the late 1950's, Dr. William Youden (1900-1971) developed what has now become known as the Youden Plots. This statistical technique involves both normal and abnormal controls and graphically helps to differentiate between systematic and random errors.
- The inner square of the plot (yellow) represents one standard deviation (1SD). The next larger square (green) represents 2SD, and the outer square (blue) represents 3SD.
- A horizontal median line is drawn parallel to the X-axis and a second median line is drawn parallel to the Y-axis.
- The intersection of the two median lines is called the Manhattan Median. One or two 45-degree lines are drawn through the Manhattan Median.
- The results of at least two different levels of controls (e.g. Level 1/Level 2 or Normal/Abnormal) are then plotted on the chart as X-axis versus Y-axis.