1178 See USPAP, Competency Rule: Acquiring Competency, comment (“Competency can be acquired in various ways, including, but not limited to, personal study by the appraiser . . . or retention of others who possess the necessary knowledge and/or experience.”). For topics that often require additional expertise in appraisals for federal acquisitions, see Section 1.13.opinion.” 1179 But the appraiser cannot merely assume such supporting experts’ reports are accurate and reliable. Rather, the appraiser must review all supporting opinions and can rely on or adopt them only if the appraiser determines, after review, that all supporting opinions are credible, reliable, and factually supported. 1180 As the Tenth Circuit succinctly stated: “[Any expert] opinion . . . must be founded upon substantial data, not mere conjecture, speculation or unwarranted assumption. It must have a rational foundation.” 1181 An appraiser who fails to ensure the rational foundation of all supporting opinions and other underlying assumptions will be left with an “ultimate opinion of value [that] is virtually devoid of factual moorings, depriving it of virtually any evidentiary value.” 1182