Google PRINCIPLES OF MOBILE SITE DESIGN: DELIGHT USERS AND DRIVE CONVERSIONS - Manual (Page 30)

18. DESIGN
EFFICIENT FORMS
Make sure your forms have no repeated actions, only as
many fields as necessary, and take advantage of autofill.
With multi-part forms, let your users know what’s
coming with a progress bar on top. Participants were
intimidated by a multi-part form with a very complex
initial step, but felt much more comfortable with a
different, more straightforward form that clearly labeled
each upcoming section. Participants also appreciated
multi-part forms that prefilled information they had
already entered, like their name and zip code.
Key Takeaway
Minimize the number of fields in your forms,
and autofill information wherever possible.
Use clearly-labeled progress bars to help users
get through multi-part forms.
Example from Progressive Mobile Site.
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