13.1 Crossed Designs and Nested Designs
Crossed designs are those in which each operator measure all parts several times. These are the most common designs, especially if dimensional features are measured.
Scheme of the crossed design (all operators measure all parts)
In a nested design, however, each part is measured by only one operator; being the type of design used in destructive tests. In nested designs, each operator should also measure each part several times, but in this case this is clearly not possible. Thus, an alternative solution is that each operator takes measures of several parts, the more alike as possible (i.e. parts produced under the same conditions); such that the heterogeneity of these parts is negligible and, hence, can practically be considered as a single part.
Nested design (each operator measures only some of the parts):