The judgment of the district court upholding the denial of plaintiff's application for supplemental security income benefits by the Social Security Administration is reversed and the case is remanded with instructions to vacate the denial of benefits and remand for further administrative proceedings, where the administrative law judge erred by not conducting a function-by-function analysis, by ignoring without explanation plaintiff's moderate limitation in her ability to maintain her concentration, persistence, or pace, and by determining plaintiff's residual functional capacity before assessing her credibility.