The Department of Justice's Office of Immigration Litigation ("OIL")--the office that defends BIA decisions in the federal courts of appeals--recently released data on federal court decisions concerning credibility.  Most credibility determinations involve asylum cases, where the decision maker (the Board of Immigration Appeals on appeal and the Immigration Judge in the first instance) must assess the alien's credibility to determine whether to grant asylum.  Under existing law, a decision maker must give a specific, cogent reason for rejecting an alien's testimony.  The REAL ID Act of 2005 tightened these requirements.  Among other things, the REAL ID requires corroboration of an alien's testimony in certain circumstances.

OIL's Adverse Credibility Project tracks appeals court decisions concerning credibility.  OIL describes the data used for the study:

The data... reflects a tally of all decisions in which - regardless of the ultimate outcome of the petition for review - the appellate court has either approved of, or reversed, the adverse credibility holding reached by the immigration judge or Board of Immigration Appeals.

The report found that the "adverse credibility win percentage in 2009 roughly parallels the overall OIL win/loss trends from that year, though the overall numbers are slightly less favorable across the board."  For all federal appeals courts in 2009, 87% of credibility determinations were upheld and 13% were reversed.  This represents a small change from 2008, during which 83% of credibility determinations were upheld and 17% were reversed. 

The overall numbers mask some diversity between the different circuit courts.  In the Second, Fourth, Fifth, and Eleventh Circuits, approximately 96% of credibility determinations were upheld.  The Sixth Circuit upheld 100% of EOIR's credibility determinations (according to OIL, the Sixth Circuit reviewed 58 cases involving credibility determinations; all were upheld).  The Third Circuit upheld 74% of credibility determinations, and the Ninth Circuit upheld 73%.  In the Eighth Circuit, 86% of credibility determinations were upheld.  According to OIL, the other circuit courts--the First, Seventh, and Tenth--heard no cases involving credibility determinations.  This seems a bit odd, especially in the Seventh Circuit, which seems to review many BIA decisions.

The circuit courts with the most changes in "win" rates for OIL are the Second and the Ninth:

Reflected in the 2009 statistics is the continued rise in win percentage within the Second Circuit. This percentage has risen steadily, from 14% in 2006 to 54% in 2007 to 90% in 2008 to 96% in 2009. Ninth Circuit win percentage, historically close to 60%, has risen in 2009 to 73%. This increase may be due, in part, to a greater percentage of post-REAL ID cases on the docket in 2009.

The bottom line, I suppose, is that it is difficult--and sometimes almost impossible--to reverse the agency's credibility determinations.  Hopefully, this is a reflection of the BIA's increased competence at adjudicating aliens' credibility, and not simply a result of stricter laws concerning federal court review.  I guess that is a possibility, but I have my doubts.