You can predict how long tasks/projects will take you (stopwatch and/or calendar time). Even if calibration doesn't generalize, it's potentially useful on its own there. And while you can't quite mass-produce questions/predictions, it's not such a hassle to rack up a lot if you do them in batches. Malcolm Ocean wrote about doing this with a spreadsheet, and I threw together an Android todo-with-predictions app for a similar self experiment.