Impact Value vs Percentage

[Gambler844]

How does one calculate Impact Value? I always see people measuring things in percentage and impact value. Why does one need both? Thanks for any help.

[George]

Gambler,
Impact value is a measure of how much something is occurring above "normal". For example in a field of 8 horses it in theory would be "normal" for each post position to win 12.5 percent of the time. (8 * 12.5 = 100). If post position one were winning 25% of the time it's impact value would be 2.00 (25 / 12.5 = 2.00). In other words it wins twice as often as it should.

IV can be very useful in creating "weights" for various factors and then summing the weights to create a relative value for each horse.

Most systems that use IV limit the maximum value for a single factor to 3.00 to reduce the impact of one abberant factor. That is done because the high "impact" is probably temporary and as Carl says will regress to the mean over time.

I distrust IV's because the definition of "norm" is sometimes arbitrary.