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Avoiding blunders involving 'immortal time'.

For almost two centuries, teachers have warned against errors involving what is now called ‘immortal time.’ Despite the warnings, and many examples of how to proceed correctly, this type of blunder continues to be made in a widening range of investigations. In some instances, the consequences of the error are less serious, but in others the false evidence has been used to support theories for social inequalities; to promote greater use of pharmaceuticals, medical procedures and medical practices; and to minimize occupational hazards. 
 
We use a recent example to introduce this error. We then discuss: (i) other names for it, how old it is and who tried to warn against it; (ii) how to recognize it, and why it continues to trap researchers; and (iii) some statistical ways of dealing with denominators measured in units of time rather than in numbers of persons.