This paper describes the main quality control methods for determining human observer measurement error and instrument error focusing on intra-observer and inter-observer technical error of measurements (TEMs) and relative TEM, and the coefficient of reliability (R). R values above 0.95 are indicative of small errors. To compare variances between different variables, the coefficient of variation, a measure of relative variability, is used. The total variation of a character can be partitioned into genetic, environmental, and error components. Determination of the genetic component of variation (heritability) is usually obtained from twin studies or other family studies. A good environment improves the population mean, whereas a poor environment can lower the population mean without any change in the underlying genetic structure. Worked examples of how to calculate TEM, R, and heritability are provided.