  In Reliability Theory, uncertainty is measured by the Shannon entropy.
Recently, in order to analyze the variability of such measure, varentropy has
been introduced and studied. In this paper we define a new concept of
varentropy for past lifetime distributions as the variability of uncertainty in
past lifetime. We analyze the relation of this varentropy with the
corresponding past entropy and other concepts of interest in reliability and we
study the effect of linear and monotonic transformations on it. Finally, we
evaluate past varentropy for random variables following a proportional reversed
hazard rate model.
