Agree with Crash and Mooee.
Odds offered by bookmakers or on TAB do not in any way reflect the REAL chances of a horse winning a race.
All they reflect is popular opinion...which is often wrong and is the only reason that punting on horses can exist.
There are so many variables that affect the outcome of a race that the realistic odds are impossible to frame. I've always thought that Bookies get a free ride when they can quote 6/4 in a 12 horse field. TAB pools..the odds on offer and subsequently the punters are influenced by the media, pre-post odds and bookmakers laying off.
Best bet in my opinion is to follow a strong stable or a horse in that stable and using a good staking plan bet on the fact that owners want a return on their training costs and initial outlay.
All the rest (weight handicapping, ratings, percentages) is peopledeluding themselves that there is an easy way of making money...never has been, never will be...usually just another case of mathsturbation.