In short, OPM does not offer specific guidance to agencies on administering additional assessments after a certificate is issued. Please check your agency's policies on administering post-certification assessments because policies may vary by agency.
To be placed on a Certificate of Eligibles, applicants must have already been screened for minimum qualifications and been assessed for critical competencies established for the position. Once a certificate is issued, the selecting official may select from any of the candidates in the highest quality category, with the exception that a non-veteran may not be selected over a veteran without an approved pass over.
However, one "post-cert" option for selecting officials is to conduct a "selecting official's interview," which may be used to verify candidates' qualifications for a job after they have been rated using other assessment methods, but prior to making a hiring decision (refer to the introduction of OPM's Structured Interview Guide, http://www.opm.gov/policy-data-oversight/assessment-and-selection/structured-interviews/guide.pdf). In a selecting official's interview, candidates' responses are not typically scored.
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