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If you have ever placed a sports bet, your chances of winning are slimmer than you might expect.The Hearst Television Data Team spoke with a statistics professor about which sports bets are least likely to win and why people should avoid them.Sportsbooks offer numerous betting options, but the most common are moneyline, spread and totals bets, according to Ron Yurko, a Carnegie Mellon University assistant teaching professor and director of the Carnegie Mellon Sports Analytics Center.Yurko is introducing a course next spring to teach students about sports betting and how sportsbook probability works. "I was motivated by this from my time as an instructor of literally seeing students on campus talk about sports betting, engage in sports betting on their phones in the classroom," he said. The course is being offered to freshmen with the hope of reaching students early in their decision-making. "These are students that are just starting their lives on their own, and they have access to a sportsbook wherever they go via their phone. And my view was we should tackle this head-on," Yurko said. What are the common sports bets? Yurko says there is one type of popular bet that has a problematic design. “One of the types of bets that really do have a low probability of winning are parlays, or even more specifically, same game parlays,” said Yurko.A parlay combines two or more bets, also referred to as a leg, into a single bet. In order to win a parlay, all of your legs have to win. If you place a three-leg parlay, but only two of your legs win, the entire parlay loses.Parlays tend to offer higher payouts, which entice bettors to take their chances on these risky bets. People often assume that if two or more events have a high percentage of happening, then the likelihood of all of them happening together is also high. But the math works differently than when calculating the probability of one event.Statisticians use joint probability to calculate the chances of multiple events occurring together, and those probabilities are often much smaller. Joint probability is the likelihood of more than one event occurring at the same time. In simple terms, it's the intersection of two or more events. To understand how joint probability works in parlays, it helps to understand how sportsbooks design and calculate American odds. How do American odds work? The American odds system may seem confusing at first, with its plus and minus signs paired with unfamiliar numbers.The plus and minus signs indicate whether a team is a “favorite” or an “underdog.” A plus sign means the team is less likely to win. A minus sign represents that the team is likely or expected to win.The numbers next to the plus or minus sign show the amount someone could stand to win or need to risk, based on a $100 baseline.If a team has +110 odds, it means you would have to wager $100 to profit $110. That would give you a total payout of $210 (your original wager plus your winnings).With -110 odds, you would have to wager $110 to profit $100. If the bet is successful, the payout would be $210. What does joint probability have to do with parlays? Once multiple events need to occur for a bet to win, the chance of winning drops significantly. Your bet is now relying on joint probability, and the more events you add, the smaller your overall chances become. Because a parlay relies on multiple independent events happening, it’s possible to calculate the odds of all those events transpiring. How bad are parlay probabilities? Even if someone is feeling lucky and is willing to take low-probability bets, that is still not the number to beat. Sportsbooks hold an "edge," meaning a bettor has to win more often than the calculated odds suggest. Put simply, it's a built-in profit margin that ensures sportsbooks make money. A sportsbook's edge can range from 4.5% to 6.5%, sometimes even higher than that if sportsbooks are not confident in their probability for pricing, according to Yurko.The odds assigned to teams do not reflect the actual likelihood of winning; it’s a number sportsbooks assign, Yurko explains. Those plus or minus odds can be converted to “break-even percentages,” which tell a bettor how many times their team has to win for them to break even and not lose money.If a team has +146 odds, it means the team has to win at least 40% of the time for a bettor to break even. However, for this same bet, sportsbooks predict that the team will win about 36% of the time.Because of that, the likelihood that a parlay is a profitable bet is less than the probability of it happening. If you think you have mastered the math, try your luck building your own three-leg parlay using the tool below.