What is the test for credibility of an immigration judge?

Although an immigration judge's credibility findings are granted substantial deference by reviewing courts, a trier of fact who rejects a witness's positive testimony because in his or her judgment it lacks credibility should 'offer a specific, cogent reason for [his or her] disbelief.'

Id. (quoting Turicos v. INS, 821 F.2d 1396, 1399 (9th Cir.1987)). Moreover,

[w]hen the Immigration judge provides specific reasons for questioning a witness's credibility, this court may evaluate those reasons to determine whether they are valid grounds upon which to base a finding that the applicant is not credible.

Id. (quoting Vilorio-Lopez v. INS, 852 F.2d 1137, 1142 (9th Cir.1988).