What the research actually says about building a habit, in plain words and with the studies named. Each one opens with a direct answer to the question in its title.
A median of 66 days in the one field study that measured it, and a range from 18 to 254. Where the 21-day figure came from.
Small enough to do on the worst day of your month. Why that is the mechanism rather than the compromise.
Naming when and where you will act roughly doubles the odds of doing it. How to pick a moment that survives a bad week.
One missed day has no measurable effect. What happens on the second day is what decides the outcome.
One. The case is arithmetic rather than moral, and the numbers are worse than most people expect.
Smaller, at the same moment, tomorrow. A gap removes repetitions and cancels none of the ones you already had.