P once worked as a carpenter and general laborer on construction sites. But he stopped working after he developed degenerative disc disease, Hepatitis C, and depression. He then applied for social security disability benefits, claiming eligibility as of October 2009. An ALJ held a hearing on P’s application. The ALJ had to determine whether the former construction laborer could successfully transition to less physically demanding work. ALJs often seek the views of “vocational experts.” Those experts are professionals under contract with SSA to provide impartial testimony in agency proceedings. Many vocational experts simultaneously work in the private sector locating employment for persons with disabilities. These experts may invoke not only publicly available sources but also “information obtained directly from employers” and data otherwise developed from their own “experience in job placement or career counseling.” The ALJ asked Erin O’Callaghan to identify a sampling of “sedentary” jobs that a person with P’s disabilities, education, and job history could perform. O’Callaghan had served as a vocational expert in SSA proceedings for five years; she also had more than ten years’ experience counseling people with disabilities about employment opportunities. She listed sedentary jobs “such as a bench assembler [or] sorter” that did not require many skills. She testified that 240,000 bench assembler jobs and 120,000 sorter jobs existed in the national economy. P’s attorney asked O’Callaghan about the source of her information. O’Callaghan replied that they came from the Bureau of Labor Statistics and her “own individual labor market surveys.” P requested that O’Callaghan turn over the private surveys so he could review them. She refused as she wished to keep the surveys confidential because they were “part of her client files.” The lawyer suggested redacting the files. The ALJ interjected that he “would not require” O’Callaghan to produce the files in any form. The ALJ issued a decision granting P’s application in part and denying it in part. According to the ALJ, P was entitled to benefits beginning in May 2013, when his advancing age (he turned fifty that month) adversely affected his ability to find employment. This conclusion was based on O’Callaghan’s testimony about the availability in the economy of “sedentary unskilled occupations such as bench assembler [or] sorter.” P appealed. P contends that the expert testimony could not be substantial evidence because O’Callaghan refused to supply the supporting data. The district court denied the appeal and the Sixth Circuit affirmed. The Supreme Court granted certiorari.