Variance components

Use to evaluate sources of variation. In Minitab, components of variance are used in ANOVA and Gage R&R.

ANOVA

Variance components assess the amount of variation in the response due to random factors. Random factors have levels that are selected at random; whereas fixed factors have levels that are controlled by the experimenter. For example, you conduct a study on the effect of two levels of pressure on output measured by randomly chosen operators. Pressure is fixed (2 levels); and operator is random. The variance components output lists the estimated variance for the operator and error term.

Gage R&R

Variance components assess the amount of variation contributed by each source of measurement error, plus the contribution of part-to-part variability. The sum of the individual variance components equals the total variation. Sources of measurement error are:

·    Repeatability – the variability from repeated measurements on the same part by the same operator

·    Reproducibility (which can be further divided into operator and operator-by-part components) – the variability when the same part is measured by different operators

Ideally, differences between parts should account for most of the variability; and variability from repeatability and reproducibility should be very small.