Yes, exactly! I'd have assumed all inpatients would effectively have a box (or a series of boxes) that could be ticked at the time of their treatment - "Covid pos or not" , and "treatment due to covid symptoms, or not, or undetermined". But it sounds that they basically had to go back through all records over a few months, and get a doctor to assign where each patient stood, according to their ward, symptoms and treatment given. Only 4 boards had the resources to do that, and each board did it separately, with in some cases only one clinician looking over the data, without review/ crosscheck etc. It just sounds so much more complicated, and with less rigour, than it maybe should be!
I appreciate it's not always going to be clearcut. It may be someone had covid a while ago so isn't showing positive (and may not have even noticed at the time), but their treatment could well be purely as a result of the covid. Similarly, someone could have covid and also a respiratory infection, or heart issue or something unrelated to the Covid but assumed to be linked just because of the timing and nature of the symptoms. One of the boards in the PHS study had a load of people from a care facility which had had a covid outbreak (seemingly) so they were all recorded as being treated "because of" covid, which meant in that board 77% of inpatients were in that category, even though they were just taken in for observation. 2 of the other boards were much closer to 50% because of/ 50% with.
But overall, the individual uncertainties aren't that relevant. As you say, people are really concerned about 1) how many people who get covid need to be hospitalised because of it. And also, 2)whether that number is going to affect services so much that other things are made less efficient (a and e times, elective surgery, cancer screening and treatment...). Obviously, 1) is now "not that many" (technical and useful term
) and not 100% of the currently declining hospital numbers, but 2) is more complicated, and not totally linked to the prevalence of the disease, but rather the measures in place to limit that disease...