if seriously injured patients admitted to the intensive care unit (ICU) were offered tea and crumpets (T&C) on day 3. Subsequent analysis would likely find that T&C recipients not only have lower mortality, but fare better overall than non-recipients. T&C therapy is thus found to benefit ICU patients! Of course it is a ruse: all we have done with T&C is to select those patients destined to do well.
Immortal time bias