In this survey, 1,207 people with migraine were asked to rate triggers on a 0–3 scale (0, never; 1, occasional, 1–33%; 2, frequent, 34–66%; 3, very frequent, 66–100%).[Kelman, 2007] As shown on the slide, stress and hormones (women) were the most common triggers reported to occur very frequently.[Kelman, 2007] The mean number of triggers per person was 6.7, with almost two-thirds of respondents reporting between four and nine triggers.[Kelman, 2007]

Reference:
Kelman L. The triggers or precipitants of the acute migraine attack. Cephalalgia 2007; 27 (5): 394–402.