of competing testimony and witness credibility. See Johnson v. Perez, 823 F.3d 701, 709-10, 2016 WL 2941965, at *7 (D.C.Cir.2016) (explaining that “[t]o the extent the testimony of a witness who is also a party may be impaired by [a] party[’s] self-interest, it is ordinarily the role of the jury—not the court on summary judgment—to discount it accordingly”); see also Desmond v. Mukasey, 530 F.3d 944, 965 (D.C.Cir.2008). Yet, in other circumstances, a witness may lack personal knowledge concerning the matter about which he attempts to offer testimony, or may make a statement that is so concluso-ry—and presented without any supporting facts in the record—that it leaves the jury “in no position to assess” the veracity of his statement. Greene v. Dalton, 164 F.3d 671, 675 (D.C.Cir.1999) (<HOLDING>). Compare Ass’n of Flight Attmdants-CWA,
holding that summary judgment was appropriate where plaintiffs admissions in her deposition undermined her claims holding that to defeat summary judgment plaintiff was obliged to offer evidence indicating that persons who actually participated in her termination decision had  knowledge of her protected characteristics holding that the plaintiffs evidence of pretext which included but was not limited to her supervisors statement that she had enough of the plaintiff going to her supervisor about her was not sufficient to preclude summary judgment holding that the plaintiffs conclusory statement that the student who was hired instead of her had less experience and education could not defeat summary judgment because absent supporting facts  a jury would be in no position to assess her claim of superiority holding that threatening employee to mind her own business investigating her videotaping her without her permission and forcing her to take polygraph could not be considered adverse employment actions because they had no effect on conditions of employment