Performance on secondary tasks next to a primary task is sometimes used to assess mental workload (Ogden, Levine and Eisner, 1979). For example, the Stroop test as a secondary task has been evaluated by Gwizdka (2010).
However, a lot of effort goes into setting up these tasks, so I was wondering whether there are any open, ready-made task sets available which would be suitable for this purpose.
Some desirable characteristics I am looking for:
- One task shouldn't be considerably harder than another.
- Performing one task shouldn't take considerably longer than another.
- Overall, one task shouldn't take too long, so the amount of tasks done can also be used as a measurement of mental workload.
Ogden, G. D., Levine, J. M., & Eisner, E. J. (1979). Measurement of workload by secondary tasks. Human Factors: The Journal of the Human Factors and Ergonomics Society, 21(5), 529-548.
Gwizdka, J. (2010, August). Using stroop task to assess cognitive load. In Proceedings of the 28th Annual European Conference on Cognitive Ergonomics (pp. 219-222). ACM.
Since you mentioned the Stroop specifically, several versions of the Stroop task are available for Inquisit here.
Randall Engle's lab also maintains a set of validated working memory tasks, which are available on request to researchers. They include full and shortened versions of operation-, symmetry-, reading- and rotation-span for E-Prime 2.0.
Assuming I read the question correctly, any of these should satisfy the requirements.