Quantitative bibliometric indicators are widely used to evaluate the performance of scientists. However, traditional indicators do not much rely on the analysis of the processes intended to measure and the practical goals of the measurement. In this study, I propose a simple framework to measure and predict an individual researcher's scientific performance that attempts to take into account the main regularities of publication and citation processes and the requirements of practical tasks. Statistical properties of the new indicator - a scientist's personal impact rate - are illustrated by its application to a sample of Estonian researchers.