The underlying variables are crucial to the correct interpretation of the data.

Simpson's paradox isn't just a hypothetical scenario.It pops up from time to time in the real world,sometimes in important contexts.One study in the UK appeared to show that smokers had a higher survival rate than nonsmokers over a twenty-year time period.That is, until dividing the participants by age group showed that the nonsmokers were significantly older on average,and thus, more likely to die during the trial period,precisely because they were living longer in general.Here, the age groups are the lurking variable,and are vital to correctly interpret the data.