In the manuscript you mentioned by Liu, et al, starting as the 3rd paragraph below "Setting, Assumption, and Notation", it lists the notation that will be used. Table 1 shows where each of those is necessary, as does most of the rest of the publication. There are five approaches to take, depending on which one you feel comfortable with in terms of obtaining the parameters related to fluid volume. The parameters are not just the range (0-2000) and the average (500). You'll also need to factor in prevalence in both those who had the event and those who did not, which is likely not readily available. The way I'm reading it, the assumptions don't work well. Also, keep in mind, fluid volume is almost definitely not the only confounder. I'll stop there because I think that's sufficient. Bottom line, I don't believe any of those methods - or any method - can estimate anything that was not measured in any of the groups. Realistically speaking, we cannot predict an event reliably using demographics.
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