PTI – Frequently Asked Questions

Q – What is a PTI?

            A – PTI is an acronym for Production Type Index

Q – What is an index?

            A – An index is a tool that combines sire summary information or Predicted Transmitting Ability (PTA) on several different traits into one measure. It is a linear combination of phenotypic (how the goat performs such as milking, or looks, such as appraisal) information and weighting factors used for genetic prediction when performance data comes from generally similar contemporary groups, such as by breed, managed in herds.  Putting all the numbers from milk testing and appraisal into two ranking numbers makes it easier than trying to use all the data.  Selection indices usually aim to identify a set of animals which carry the combination of genes that is most likely to achieve the objective across the whole herd.

Q – How is it used?

            A – To assist in selecting breeding which is why it is also called a selection index.  In an ideal situation it would be desirable to select animals for breeding that excel in all traits.  But, in reality it is always necessary to make some compromises in balancing the strengths and weaknesses among the animals available for selection. A selection index is a formal method of combining Predicted Transmitting Ability (PTAs) for different traits into a single PTI ranking. 

Q – How many indexes does ADGA have?

            A – There is no limit to what information can be combined into an index.  Currently, ADGA has two and they both combine production and appraisal information.  One emphasizes milk production twice as much as type (the “2:1”) and the other which reverses the emphasis, giving more weight to type (the “1:2”).

Q – What information goes into the PTI calculation?

            A – Predicted transmitting ability (PTA) for fat corrected milk & PTA for final score adjusted by breed averages for fat and standard deviations for fat corrected milk and type by breed.  This is expressed in lbs. for production.  For type, the standard deviation (SD) is a measure of the degree of spread around the average value.  Most values will be between -3 and +3.   So the index combines fat%, amount of milk plus all of the linear traits as they go into the final score into a single ranking.

Q – What is fat corrected milk?

            A – A way to standardize milk yield in pounds so that breeds with low butterfat can be on equal footing with breeds with high butterfat percentages.  For example, if Betsy the Nubian produces 1500 pounds with 5% butterfat and Clara the Toggenburg produces 2000 pounds with 3% butterfat, we could fat correct their milk to 4% and say that Betsy had 1725 pounds and Clara had 1700 so that we can compare them equally.

Q – How does a goat get a PTI?

            A – Because an animal needs to have fat corrected milk and type information in order for the calculation to be performed, it must have both DHI information to get a PTA for milk and fat and LA information to get a PTA for type.

Q – How then do bucks get milk PTA?

            A – On the basis of how their daughters perform.  The can get a PTA for type when three daughters have been appraised, but it takes 5 daughters on test in order to get a PTA for milk production.  When a young buck without daughters has parents with all of the information, we can estimate the transmitting ability and get the ETA.

Q – What does a PTA number mean?

            A – PTA is expressed in pounds or score either positive or negative as compared to the average.  Average PTA would be “0” neither transmitting the ability to milk more or less, or to have average final scores of more or less than average.  A Milk PTA of 233  would mean that we expect daughters of that buck to produce 233 pounds of milk more than the average. 

Q – Do PTAs and PTIs stay the same?

            A – No, they are adjusted each time a genetic evaluation is performed at CDCB.  Records up to the 6th lactation are used in the Production evaluation for the individual animal, and for the Type evaluation, all scores no matter what age are used.  Also, each genetic evaluation takes in all immediate relative’s performance as they are added into the database. 

QWhen are evaluations done?

            A – Production evaluations are done twice a year in August and December.  Type (from linear scores) is done once in December after all scores for the year have been entered into the database.

Q – What does the PTI number mean?

            A – It is a ranking.  Higher PTIs mean there were higher PTA values contributing to the index calculation.  The rankings can only be used to compare animals within the same breed.

Q – What does REL – reliability add to the information?

            A – This is the measure of confidence in a PTA.  Reliability is calculated by the amount of information used in the evaluation. Information from the animal, parents and progeny are considered. Lower reliability means the PTA is more likely to change when a significant amount of information is added to the evaluation. As the amount of information on the animal and relatives increases, REL also increases.  40-70% is considered to be moderate reliability.

Q – Why are some PTIs negative?

            A – Positive values mean increased PTAs which translate to an economic profit (more milk = more money), negative values mean a reduction in PTA and reduction in profitability or conformation.



Holstein – Understanding Genetics and the Sire Summaries

USDA – Genetic Evaluation Program