A brief review of the concepts and methods involved in the objective description and measurement of "personality" by means of interaction chronograph methods, together with a comment on previous studies, have been presented.The present study was concerned with testing the reliability of lO-min. and 30-min. samples of Period I type behavior, following the rules of a partially standardized interview. Using interaction chronograph measures, it was concluded that 10 min. are sufficient to reach a stable pattern of patient communication under Period I conditions. It was established further that extending the period of observation to 30 min. offers a much more stabilized interaction.