The Cost of Sequential Adaptation and the Lower Bound for Mean Squared Error

Informative interim adaptations lead to random sample sizes. The random sample size becomes a component of the sufficient statistic and estimation based solely on observed samples or on the likelihood function does not use all available statistical evidence. The total Fisher Information (FI) is decomposed into the design FI and a conditional-on-design FI. The FI unspent by the interim adaptation is used to determine the lower mean squared error in post-adaptation estimation. Theoretical results are illustrated with simple normal samples collected according to a two-stage design with a possibility of early stopping.