From The Washington Post:
At 11:26 p.m. Monday, a laid-off journalist sent out a single tweet that would ignite the media and force the Trump campaign onto the defensive at the Republican National Convention. Melania Trump’s speech, he found, was eerily similar to one given by Michelle Obama at the Democratic National Convention eight years earlier.
The Trump campaign and supporters gave a broad range of responses before a Trump Organization staffer announced she was responsible for the similarities. One of those responses was that it was a coincidence that Melania Trump and Michelle Obama had such similar ideas. Another was it’s not plagiarism when only 7 percent of the speech is the same. A third was it’s Hillary Clinton’s fault.
The argument that it was purely a coincidence was always on slim ground. At least statistically speaking, the Trumps didn’t have basic probability on their side. Indeed, it’s less likely she and Obama independently wrote these speeches than if she were struck by lightning. Twice. (I’m not the only one to think of this comparison.)
To understand the math, let’s begin with a calculation that gives Melania Trump the benefit of the doubt. Let’s assume she actually did have the same ideas, and express them in the same words, as Obama.
Even then, as a McGill University astrophysicist described in a Facebook post, the chances that Trump and Obama’s speeches would come out so similar is near-zero. There are, by his count, 14 distinct copied phrases between the two speeches — phrases like “willingness to work for them” that could be reordered in almost any configuration and still make a coherent speech.
In reality, not all of these orderings are created equal — some may create a more logical flow than others, giving them a higher probability to be chosen. So the estimates below should be treated as a ballpark …