4 C.F.R. § 28.61
(a) In appealable actions, as defined by 5 U.S.C. 7701(a), agency action must be sustained by the Board if:
(b) Notwithstanding paragraph (a) of this section, the agency's decision shall not be sustained if the petitioner:
(d) Definitions. For purposes of this section, the following definitions shall apply:
Harmful error means error by the agency in the application of its procedures which, in the absence or cure of the error, might have caused the agency to reach a conclusion different from the one reached.
Preponderance of the evidence means that degree of relevant evidence which a reasonable person, considering the record as a whole, would accept as sufficient to support a conclusion that the matter asserted is more likely to be true than not true.
Substantial evidence means that degree of relevant evidence which a reasonable person, considering the record as a whole, might accept as adequate to support a conclusion, even though other reasonable persons might disagree. This is a lower standard of proof than preponderance of the evidence.
[58 FR 61992, Nov. 23, 1993, as amended at 68 FR 69302, Dec. 12, 2003]