Based on cognitive load theory, this paper reports on two experiments investigating the variability effect that occurs when learners' exposure to highly variable tasks results in superior test performance. It was hypothesised that the effect was more likely to occur using high rather than low levels of guidance and testing more knowledgeable than less knowledgeable learners. Experiment 1, which tested 103 adults studying pre-university mathematics, showed no interaction between levels of variability (high vs. low) and levels of instructional guidance (worked examples vs. unguided problem solving). The significant main effect of variability indicated a variability effect regardless of levels of instructional guidance. Experiment 2, which tested another group of 56 adults enrolled in the same mathematics program, showed an interaction between levels of variability (high vs. low) and levels of learner expertise (novices vs. experts). More experienced learners learned more from high rather than low variability tasks demonstrating the variability effect, while less experienced learners learned more from low rather than high variability tasks demonstrating a reverse variability effect. It was suggested that more experienced learners had sufficient available working memory capacity to process high variability information while less experienced learners were overwhelmed by high variability and learned more using low variability information. Subjective ratings of difficulty supported the assumptions based on cognitive load theory. The major educational implication is that learners should initially be presented with low variability or easier tasks, and as they gain more experience in the task domain, variability or task difficulty should increase.