How are the odds for scratch-off tickets determined?

It is the total number of tickets printed for a game divided by number of winning tickets printed.
For instance, if there are 400,000 winning tickets for a game and there are 1.4 million tickets printed, the odds are: 1.4 million/400,000 = 1: 3.5. So, on average (for this example), the odds of winning are 1 in every 3.5 tickets. That doesn't mean that every 3 or 4 tickets is a winner. It means that out of all the tickets printed, the average number of winners will come out to 1 in 3.5. In some cases, there will be a number of winning tickets in a row and, in other cases, there may be a number of tickets in succession without a winner.