**Tester stratification**

Test stratification is to generate subset of testers based on their placements in the biplot. The following options are available:

**Testers with long vectors**

When this function is invoked, GGEbiplot first calculate the vector length for each of the testers and express them relative to the longer vector, and then ask the user to set a criterion, against which testers with LONGER vectors are selected and retained. A new biplot based on the selected subset of testers will be generated.

**Testers with short vectors**

When this function is invoked, GGEbiplot first calculate the vector length of all testers and express them relative to the longer vector, and then ask the user to set a criterion, against which testers with SHORTER vectors are selected and retained. A new biplot based on the selected subset of testers will be generated.

**Testers within a sector**

When this function is invoked, GGEbiplot first calculates the angles of the testers, relative to an arbitrary reference point. It then asks the user to set a sector defined by an upper and a lower bound; all testers WITHIN this sector will then be retained and a new biplot generated.

**Tester outside a sector**

When this function is invoked, GGEbiplot first calculates the angles of the testers, relative to an arbitrary reference point. It then asks the user to set a sector defined by a upper and a lower bound; all testers OUTSIDE this sector will then be retained and a new biplot generated.