Wood (1987) found in his research a log-linear relationship (R = 0.94) between
hardness tested with the Kahl device and durability tested with a Holmen pellet tester.
These relations may be found within certain diet formulations. However, because of the
fact that hardness and durability are two distinct quantities, differences in relationships
may be found between different diet formulations. These differences may become more
distinct when new types of equipment are used to manufacture pelleted feed, e.g.
expanders.