A major problem with current measures of cognitive effort is the confounding of effort and difficulty. This is hampering efforts to understand individual differences in effort sensitivity because, with these tasks, results could equally well be explained by differences in effort sensitivity or cognitive capacity. To address this shortcoming we introduce a new test, the Number Switching Task (NST), specially designed such that difficulty will be unaffected by the effort manipulation and can easily be standardised across participants. In a large, online sample we show that these criteria are met successfully and reproduce classic effort discounting results with the NST. We also demonstrate the use of computational modelling with this task, producing behavioural parameters which can then be associated with other measures, and report a preliminary association with the Need for Cognition scale. Our write-up for this study has been posted as a preprint to PsyArXiv (https://doi.org/10.31234/osf.io/rbwmp). This study was preregistered here on OSF (https://doi.org/10.17605/OSF.IO/8Y7P9). Data and analysis code from this study are available to download from this repository. The Number Switching Task itself is available to view on the Gorilla open materials repository (https://app.gorilla.sc/openmaterials/328049).