I hope you don't teach statistics
Do you? Stats are easily manipulated to achieve almost any conclusion, seen it done and seen it give credence to more than one side of an argument, but that has little to do with this;
Sure the nomination race can be a bit of a crap shoot but the strongest candidates usually show themselves early. There are only ever two viable candidates for the presidency. Betting on the incumbent for a second term is almost a sure bet given George H.W. Bush was the most recent incumbent and one of only four sitting presidents to lose. Ross Perot was the wild card in that one. Carter being beaten by Ronald Reagan was almost a given, I still remember the joke from high school, "What's flat, black and glows in the dark? Iran after Reagan is elected".
Some are too close to call, like George W. and Al Gore, but there have been quite a number of decisive victories in recent past.