The added reinforcement of variable responding will facilitate the learning of a difficult target sequence in rats, however, the added requirement of variability has been shown to impede difficult sequence learning in humans. The present study aimed to explore the notion of sequence difficulty in humans by manipulating sequence length (6-12 items). Eighty participants were randomly allocated to one of two groups: Control and Variable. In the control conditions sequences were only reinforced if they were the target sequence, in the variability conditions sequences were concurrently reinforced on a Variable Interval 60-s schedule if the just entered sequence met a variability criterion. For the six-item sequence (122121) the Control group were most likely to produce the target sequence, while for the twelve digit sequence (221112211121) there was no difference between the two groups. The Variable group were most likely to produce the target sequence for the intermediate nine-digit sequence (112212121).The use of sequence length as a definition of sequence difficulty in both the current and previous studies are discussed.