Two groups of human subjects pressed buttons on five different variable-interval (VI) reinforcement schedules presented for seven minutes each for 15 sessions. At session 16, the same VI schedules were programmed concurrently in each session either with or without a 5 s changeover delay (COD). The same schedule-correlated stimuli were employed in single and concurrent conditions. Two other groups responded on concurrent VI VI conditions from the first session with or without the COD. Response allocations under concurrent scheduling better approximated relative reinforcement frequencies when the COD was programmed. Subjects with single VI histories failed to match response and time allocations to reinforcement ratios better than subjects given no such history. Bidirectional cumulative records are discussed as a molecular data analysis technique.