Possible Change

Organizations involved in producing EPDs will often provide a table of "possible change" values. Possible change is a measure of the potential difference between an EPD and the true progeny difference. Possible change values are different for each trait because they are constructed using traits' genetic variances. Also, possible change values are different at each level of accuracy.

When calculating the possible change table values, inbreeding is assumed to be zero. The possible change values should be calculated from the following expression:

$PC={t}{\frac {(1-a_{BIF})\sigma _{u}}{2}}$

where PC is a possible change value for a corresponding BIF Accuracy value ($a_{BIF}$); $\sigma _{u}$ is the additive genetic standard deviation for that trait; and, t is either 1 or 2 depending on the desired width of the confidence interval. Some organizations publish possible change values as one EPD standard prediction error (t=1). One standard prediction error means the true value should fall within the range of plus or minus one possible change from the EPD 68% of the time for a given level of BIF Accuracy. Other organizations choose to publish possible change values as two standard prediction errors (t=2) and the true value will fall within plus or minus the possible change value 95% of the time.

Recommendations

BIF recommends that possible change tables should be clearly labelled as representing 68% or 95% confidence ranges