Importance of latent variables

Strangely enough, Hospital B is still the better choice,with a survival rate of over 98% So how can Hospital A have a better overall survival rate if Hospital B has better survival rates for patients in each of the two groups?What we've stumbled upon is a case of Simpson's paradox,where the same set of data can appear to show opposite trends depending on how it's grouped.This often occurs when aggregated data hides a conditional variable,sometimes known as a lurking variable,which is a hidden additional factor that significantly influences results.Here, the hidden factor is the relative proportion of patients who arrive in good or poor health.