Why do we not consider sickness a holding state? Is it due to the short time period (an hour) that we are there for?
Hi Sounds like a lovely disease doesn't it? You are right that there is a brief window when we don't know if someone will recover or not, and both the Examiners Report and ASET does discuss this possibility as a valid alternative solution to the question. Hope that helps Dave
Thanks Dave ! One more question... On part (iv), when calculating PHR, why has it been decided to calculate via integrated equations. Would it be possible to solve in any other way?
Hi The alternative method for generating explicit equations for these probabilities is to solve the Kolmogorov differential equations. I haven't attempted to do that for this question, but I suspect it would be a lot harder since this is generally only feasible in simple cases. Generally speaking, integral equations work best when we are considering specific paths through the process, which you can spot because you can't write them down in the usual notation (eg "find the probability that someone who has never had the disease catches it then dies from other causes within 5 years"). That is not the case here, since we can write 3_p_0^HH + 3_p_0^HR, so it is not clear that the examiners were looking for an integral equation. This whole question leaves open several routes you could follow, which would be hard under the stress of an exam. More recent questions on this topic have been clearer, so I recommend you give those a go and don't spend too much time on this one. That said, it's good practice for the sort of curveballs the examiners might throw at you. Dave
