Data from a study on perception of mental control and mental effort. The dataset sorts participants into the condition they were in (syntactic, semantic, or control, across the mental constructs believing, focusing, inhibiting, deciding, seeing, visualising, and remembering), and records the timing of their response, and their answers to our questions, described above. The columns are named according to the experimental condition. Answers to the extent, control, and effort questions are reported on a 1-100 scale.