testimony seemed confused. Based on these facts, Blankenship argues that the district judge should have declared Miles to be incompetent and should have refused to allow her to testify against him. The trial court has complete discretion to decide whether a witness is competent to testify. See Gurleski v. United States, 405 F.2d 253, 267 (5th Cir.1968), cert. denied, 395 U.S. 977, 981, 89 S.Ct. 2127, 2140, 23 L.Ed.2d 765, 769 (1969). Moreover, the presumption is that “[e]very person is competent to be a witness” if that person has personal knowledge of a matter and states that she will speak truthfully. See Fed.R.Evid. 601. The flaws in Miles’s testimony may have impaired her credibility, but they did not affect her competency. See United States v. Ramirez, 871 F.2d 582, 584 (6th Cir.) (<HOLDING>), cert. denied, — U.S. -, 110 S.Ct. 127, 107
holding that because the line of questioning was not relevant to the credibility of the witness and sought information about which the witness had no knowledge it was not a constitutional violation to sustain the objection holding that error is preserved regarding a defendants right to confront a witness if the defendant identifies for the trial court the subject matter about which he or she desired to examine the witness and the subject had a tendency to affect the witness credibility holding that promises made by the prosecution to a witness in exchange for that witness testimony relate directly to the credibility of the witness holding that witness drug use at the time of the incidents about which he was testifying went to credibility not competency holding that evidence of a threat to a witness by the defendant is relevant as showing an attempt to prevent a witness from testifying and avoid punishment for the crime