The reference versus bias plot consists of

·    Plotted points, which represent the bias (the y-axis) and the reference part measurements (the x-axis).

·    The red symbols, which represent the average of the deviations from the master for each part.

·    The line, which is a least squares regression line fit to the average of the deviations.

Ideally you want the deviations for each part to be close to 0 and the line to be horizontal.

For the parts data, there appears to be a linearity problem. Measurements for smaller parts are higher than their corresponding reference part measurements, while measurements for larger parts tend to be lower than their corresponding reference part measurements.