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I've done some research and noticed that a large number of successful eCommerce sites use a drop-down box for the 'expiration date' and other sites use a small text-box that allows for 2 or 4 characters with 'MM' or 'YYYY' greyed out.
What do you think is better UX-wise? Drop-down or small text-box? Do you have any research/case studies/AB tests to back this up?