One of the most common ways to keep track of boxed combinations is to list the individual digits from lowest to highest, from left to right: the straight number 309 is rewritten as 039, 972 is rewritten as 279, 744 as 447, etc...
Each different digit has the same chance of appearing anywhere within a straight combination (27.1%), but when the digits are listed in a low-medium-high (LMH) format for boxed tracking purposes, there are many different probabilities associated with them. The charts below illustrate how different digits are more or less probable to appear in a LMH formatted list of boxed Pick 3 combinations.
The first chart below shows that the digit 0 will be drawn as the lowest digit 27.1% of the time - or 27.1% of all games. The digit 1 will be drawn as the lowest digit 21.7% of the time and the digit 2 will be drawn as the lowest digit 16.9 % of the time. The higher the "Low" digit, the less likely it is to occur in the Low position.
In the chart below, the digits 4 and 5 are the most frequently appearing digits in the Medium position, each with 14.8% or a combined total of 29.6%. Obviously, the only way the digit 0 can be shown as the medium digit is if the combination drawn contained double or triple zeros. The same can be said for the digit 9.
The chart for the High position shows that the digit 9 is the most probable and common digit to appear in the High postion with a 27.1% chance. The probabilty for digits in the High position is an inversion of those in the Low.
All and all, these charts actually represent the different probabilities for straight combinations that contain various digits as high, low or medium numbers, i.e. there are 169 straights that contain the digit 7 as the highest possible digit or 112 that contain 2 as the medium digit etc...
Tracking high, medium and low digits for boxed play is very effective. It's interesting to note how often the LOW_HIGH pairings of 08, 09, 18, and 19 occur with such great and consistant frequency. There are many more stats on this tracking method.
~Probability=Odds in Motion~