This result is the index score and can be used to compare the worth of each organism being selected.
Thereby, no useful traits are being excluded from selection at any one time and so none will stagnate or reverse while you concentrate on improving another property of the organism.
However, its major disadvantage is that the weightings assigned to each characteristic are inherently quite hard to calculate precisely and so require some elements of trial and error before they become optimal to the breeder.
[2] The selection index theory is well described in Erling Strandberg and Birgitte Malmfors's notes under the headings Genetic Evaluation.
Calculation of a selection index based on actual data can be carried out using an applet made by Knud Christensen.