There are two types of error that one should look for when examining quality control charts. Random errors are statistical fluctuations (in either direction) in the measured data due to the precision limitations of the measurement device.

Systematic errors, by contrast, are reproducible inaccuracies that are consistently in the same direction. Systematic error is more subtle and harder to detect. Causes for this type of error could include standards and reagents being improperly prepared, or a wrong setting on the instrument.