I got interrupted, so here my post continues.

Most punters can calculate these values for anything rather simply. You need the data and a tool that will crunch that data.

Next you need to analyse a sample of data to get the over all figures of runners vs winners, or the general strike rate as that is what you are going to divide.
Once you have that then running a test over the same sample, using one form filter at a time will, via the results, give you the figures you need to calculate the impact value of that form/rating element.

Something to watch out for, is when running 2 filters at a time or more, 'to save time'. Because then you are going to get the impact value of that 'pair' of filters.

If your tool has analysis function, where you can breakdown the first test (the all figs) then you can simply obtain the strike rates from the analysis in one go rather having to run individual tests.

Another thing to be aware of, the initial ALL sample. In the sample of AWT in CP post, your ALL sample needs to be based on a filter that isolates only AWT tracks. So you need to be careful of what your starting point is and to make sure you are not using a wrong one.

Another good example would be to calculate the IV of jockeys, based on all jockeys performance % vs individual jockey performance %. Here you need to have that ALL sample and then using analysis of individual jockeys strike rate you can simply create these values in a spreadsheet if you can export.

No great mystery, and you don't even need a book! Where it does become interesting is when you start thinking about your IV. Running just blindly ALL as described before will produce IV that everyone else will produce. And that may be good enough. It is the ACADEMIC sample. You know math performed by people or programers that don't punt! However there are further ways that you can drill down to obtain more truer IV ratings, by removing the chaff or if you like horses that otherwise cloud the 'issue'.

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Michal - Ratings2Win Pty Ltd
R2W Axis - Axis is Australia's leading horse racing software and database;
with sophisticated form analysis tools and accurate horse performance ratings for TAB meetings.
http://www.ratings2win.com.au/