They don't give enough detail in the article (Wikipedia has some more https://en.wikipedia.org/wiki/Lucia_de_Berk https://en.wikipedia.org/wiki/Lucia_de_Berk),

They don't give enough detail in the article (Wikipedia has some more https://en.wikipedia.org/wiki/Lucia_de_Berk https://en.wikipedia.org/wiki/Lucia_de_Berk), but based on the chart I suspect it's something like this: Imagine someone goes to a casino, and play a game where you flip a coin 20 times in a row, and win if they're all heads. There's about a 1 in 1 million chance of flipping 20 heads - so people are suspicious that the person cheated. However, imagine that during that same time period, 10 million people played that game. Looking at that, you'd expect there to be 10 winners of the game. Suddenly it's not suspicious at all. So I think the argument is that this is the chance a nurse experienced this many deaths is 1 in 45 - so with 45 nurses, you'd expect one to experience it.