Acquire par figures. Par figures are the average times that horses run in terms of distance and classification: For example, $10,000 claiming races for horses racing 6 furlongs. You calculate par times by looking at result charts from races at a particular track and comparing the times run by horses on that day in different classifications of races to the par figures.
Calculate the par figure for each day of racing. You might average the differences from par in each race to derive the par figure of the day, or take a more subjective and sophisticated approach to determine what overall par number for the day makes the most sense.
Apply the par figures to the horses you are evaluating. For example, Horse A in a $20,000 claiming race runs 1 mile in one minute, 36 seconds on February 14 and the par for that classification is one minute, 37 seconds. The horse has run one second faster than par. One second equates to roughly five lengths. Therefore, the horse has run five lengths faster than par and his "true" figure for the race is one minute, 35 seconds.
Compare the "true" times for each horse in the race. Using the above example, let's say Horse B ran a mile in one minute, 36 seconds on February 21, when the track was rated one second faster than par time. Therefore, the "true" time for Horse B would be one minute, 37 seconds. Although both horses ran the same "raw" time, one minute, 36 seconds, Horse A is actually two seconds, or a whopping 10 lengths, faster than Horse B.