The extent to which discrimination plays a role in explaining gender wage disparities is difficult to quantify, due to a number of potentially confounding variables. A 2010 research review by the majority staff of the United States Congress Joint Economic Committee reported that studies have consistently found unexplained pay differences even after controlling for measurable factors that are assumed to influence earnings – suggestive of unknown/unmeasurable contributing factors of which gender discrimination may be one. Other studies have found direct evidence of discrimination – for example, more jobs went to women when the applicant's sex was unknown during the hiring process than when it was known. Other factors that have been speculated to contribute to the gap include a greater value placed on non-wage benefits and a difference in willingness and/or skills to negotiate salaries.