28 CFR 22.27: Notification.
(a) Any person from whom information identifiable to a private person is to be obtained directly, either orally, by questionnaire, or other written documents, shall be advised:
(1) That the information will only be used or revealed for research or statistical purposes; and
(2) That compliance with the request for information is entirely voluntary and may be terminated at any time.
(b) Except as noted in paragraph (c) of this section, where information is to be obtained through observation of individual activity or performance, such individuals shall be advised:
(1) Of the particular types of information to be collected;
(2) That the data will only be utilized or revealed for research or statistical purposes; and
(3) That participation in the project in question is voluntary and may be terminated at any time.
(c) Notification, as described in paragraph (b) of this section, may be eliminated where information is obtained through field observation of individual activity or performance and in the judgment of the researcher such notification is impractical or may seriously impede the progress of the research.
(d) Where findings in a project cannot, by virtue of sample size, or uniqueness of subject, be expected to totally conceal subject identity, an individual shall be so advised.