Cull anything that falls below your standards. Individual can be culled for lacking in only one trait, but be excellent in the other traits. A formula is set up that assigns a weighting factor for each trait being considered in a selection program. All the important traits are combined into one overall value. An index considers the relative economic importance of each trait.
Index is difficult to come up with.
Average farmer cannot develop his own index. An index determined on another group of animals may not be optimum in your herd.