How long does a habit take? What a real-world study found

Step 1 Warm-up recall

From M7.L4: write the no-effect value for d and for an odds ratio.

Your answer

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Step 2 Learning goal

By the end of this lesson you can:

• report the design and findings of Lally and colleagues (2010) (Foundation);
• explain why a range and a median say more than one number (Core);
• convert days into weeks (Core).

Step 3 A familiar example

A friend says “it takes exactly a month to make a habit”. Another says it took them half a year. A single number hides a lot of variety.

Step 4 Clear explanation

Lally, van Jaarsveld, Potts and Wardle (2010) asked volunteers to choose a new eating, drinking or activity behaviour and to do it daily in the same context for 12 weeks (84 days). They recruited 96 volunteers and 82 provided enough data for analysis. Each day participants reported how automatic the behaviour felt. The authors modelled how automaticity rose and levelled off, and reported that the time to reach a plateau ranged from 18 to 254 days across participants. The results section reports a median of 66 days for the 39 participants whose data fitted the model well. The abstract also states that missing one opportunity to perform the behaviour did not materially affect the habit-formation process.

What can you take from this? Habits did not form on a fixed schedule: some people reached a plateau in about two and a half weeks, others took much longer than the 12 weeks observed, and the 254-day end of the range comes from fitting a curve beyond the observed period. A median of 66 days is about 9.4 weeks. Your own four weeks may be too short to see a habit form, which is why this course tests whether the habit helps, not whether it has become automatic.

Limits: volunteers chose their own behaviours, the sample is small, and the median rests on 39 people with a good model fit.

Step 5 Worked example (4 marks)

A blog says “science shows habits take 66 days”.

1. 66 is the median among 39 participants with a good model fit. (1 mark)
2. The range across participants was 18 to 254 days. (1 mark)
3. 66 days ÷ 7 = 9.4 weeks. (1 mark)
4. Fair statement: in one study, the median time to plateau was 66 days, with wide variation; missing one day did not materially affect the process. (1 mark)

Step 6 Learner practice (instant-marked)

Step 7 Recap

• Lally and colleagues (2010): 96 recruited, 82 analysed, 12 weeks; time to plateau ranged from 18 to 254 days.
• Median 66 days among 39 participants with a good model fit.
• A missed opportunity did not materially affect habit formation.

Step 8 Exit ticket (ungraded)

Write one sentence reporting the time habits take without using a single number.

Your answer

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Step 9 Go further (Extension)

Read the Lally and colleagues (2010) abstract in the European Journal of Social Psychology. Find what the authors say about the shape of the curve.