There were enough additional deaths to make the doctors take notice. Some experts say that this actually wasn't a statistically significant number of deaths and shouldn't have resulted in further action. I don't really understand that tbh - surely if something feels wrong about babies dying unexpectedly, it's right to look into it? By that logic, we shouldn't have safeguarding procedures in place in any org until there's been a statistically significant number of incidents. That's not how it works. The investigation that was could have found that a faulty machine or poor process in recognising/ treating sepsis or NEC was in place.
Nobody would be complaining if a small number of deaths had alerted the hospital to this. But it didn't - the fact that these deaths were not satisfactorily explained was the answer and a bad actor was the last explanation left.
The other thing I've seen is a statistician saying that when you include all the deaths that happened in the period on the chart, Lucy's involvement would not look like it does. There are two reasons this is an issue, and both of them should really embarrass this particular 'expert'. First, there was a good reason why not to include all deaths. The deaths on there were the deaths that were not adequately explained. Sometimes sadly the death of a baby is inevitable and the cause is clear. The investigation isn't interested in these cases. So they're not on the chart. This was immediately pointed out and that's why the expert concerned ended up looking rather silly.
Now, you can argue that the expert who decided which cases are and aren't unexplained could be wrong - just like you can argue black is white about anything else- but to me at that point you are properly into I-know-better-than-the-experts-with-my-internet-research territory, I'm afraid.
The other reason why I'm not convinced about flawed stats here is that in court, the chart was there to show opportunity, not to prove each murder. There was PLENTY of other evidence for every guilty verdict to demonstrate the means. But if you accept that there was foul play going on, then you have to accept that either it was Letby or there were at least two bad actors working on the unit. So the chart was presented in that context, as a piece of the argument.
There's a lot to be gained by shouting about this on social media and sticking your head above the parapet, but I really doubt that overturning the verdict is Gill or Davis's end game here - they've got a lot of other things to win while they give this a go and get their five minutes.