Agency's construction of statute was permissible, and made considerable sense in terms of the statute's basic objectives. In addition, the agency's regulations reflected the agency's own longstanding interpretation. An individual is not entitled to a trial work period if he or she performs work demonstrating the ability to engage in SGA within 12 months of the onset of the impairment(s) that prevented the performance of SGA and before the date of any notice of determination or decision finding that he or she is disabled. Duration requirement in the definition of disability means that both the medical impairment and the inability to engage in substantial gainful activity (SGA) must have lasted or be expected to last for a continuous period of not less than 12 months.