A. Scope of Review. The findings of the Commissioner as to any fact, "if supported by substantial evidence, shall be conclusive." See 42 U.S.C. § 405(g). Substantial evidence is such relevant evidence as a reasonable mind might accept as adequate to support a conclusion. Richardson v. Perales, 402 U.S. 389, 401 (1071); Hale v. Secretary of Health and Human Services, 816 F.2d 1078, 1082 (6th Cir. 1987). It has also been defined as more than a scintilla and less than a preponderance. Hames v. Heckler, 707 F.2d 162, 164 (5th Cir. 1983); Sprague v. Director, Office of Workers' Comp., Etc., 688 F.2d 862, 865 (1st Cir. 1982). The Commissioner's determination must be affirmed if supported by substantial evidence, notwithstanding conflict in the medical testimony, and even if there was also substantial evidence which would have supported a finding in favor of the plaintiff. See Sit. v. Schweiker, 671 F.2d 19, 22 (1st Cir. 1982); Estep v. Richardson, 459 F.2d 1015, 1017 (4th Cir. 1972).
It is the function of the Commissioner and not the courts to resolve conflicts in the evidence. Richardson v. Perales, 402 U.S. 389 (1971). Aponte v. Secretary, Department of Health and Human Services, 728 F.2d 588, 591 (2d Cir. 1984); Sample v. Schweiker, 694 F.2d 639, 642 (9th Cir. 1982). Likewise, it is the Commissioner's duty to pass on the credibility of witnesses. Richardson v. Perales, 402 U.S. 389 (1971); Carroll v. Secretary of Health and Human Services, 705 F.2d 638, 644 (2d Cir. 1983); Smith v. Schweiker, 546 F.2d 1075, 1061 (5th Cir. 1981).
B. Standards For Determining Disability. In order to qualify for disability benefits under the Social Security Act, a claimant must demonstrate that he is unable to "engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or which has lasted or can be expected to last" for at least 12 months. 42 U.S.C. §§ 416(i) and 423(d)(1)(A); See also Pass v. Chater, 65 F.3d 1200 (4th Cir. 1995). Pursuant to 42 U.S.C. § 423(d)(3) "'a physical or mental impairment' is an impairment that results from anatomical, physiological, or psychological abnormalities which are demonstrable by medically acceptable clinical and laboratory diagnostic techniques." See also Greenspan v. Shalala, 38 F.3d 232 (5th Cir. 1994). Regulations promulgated by the Commissioner describe how the evidence, including medical opinions and subjective complaints, are to be evaluated. 20 C.F.R. §§ 404.1527 - 404.1529; 416.927 - 416.929; See also Diaz v. Shalala, 59 F.3d 307 (2d Cir. 1995).
C. Sequential Evaluation. The Commissioner's regulations establish a five-step sequential evaluation process for determining disability. 20 C.F.R. §§ 404.1520, 416.920. Under this process the adjudicator first determines whether the claimant is performing "substantial gainful activity" (SGA). 20 C.F.R. §§ 404.1520(b), 416.920(b). If the claimant is performing SGA, the claim is denied. Id. If not, the adjudicator proceeds to step two and determines whether the claimant has a medically severe impairment or combination of impairments, the term "severe" describing an impairment which significantly limits the claimant's ability to perform basic work activities. 20 C.F.R. §§ 404.1520(c), 416.920(c). If the claimant does not have a severe impairment or combination of impairments, the claim is denied. Id. If the claimant's impairment, or any combination thereof, is severe, the adjudicator determines at step three whether the impairment meets or equals the medical criteria of a "Listing-level" impairment that the Commissioner acknowledges is so severe as to preclude SGA. 20 C.F.R. §§ 404.1520(d), 416.920(d); see 20 C.F.R. Pt. 404, Subpt. P, App. I. If the claimant's impairment is not presumptively disabling under the Listings, the adjudicator determines at step four whether the impairment prevents the claimant from performing his/her past relevant work, i.e., SGA performed for a period long enough to have learned how to do the job within the 15-year timeframe prior to the application. 20 C.F.R. §§ 404.1520(e), 404.1565(a), 416.920(e), 416.965(a). "Work" in this context refers to the claimant's past type of job, not to the particular job itself. 20 C.F.R. §§ 404.1560(b), 416.960(b). If the claimant cannot perform his/her past work or has no past relevant work, the adjudicator proceeds to step five and evaluates whether there is other work available in the national economy that the claimant can perform in view of his/her vocational profile (age, education, and work experience). 20 C.F.R. §§ 404.1520(f), 416.920(f). The adjudicator may rely on the Commissioner's Medical-Vocational guidelines, 20 C.F.R. Pt. 404, subpt. P, App. II, to establish the existence of available jobs, provided that the claimant's residual functional capacity (RFC) and vocational profile coincide with the criteria of a particular Guideline, or "Grid," rule. If the claimant has a non-exertional impairment which significantly reduces the range of jobs available under the applicable Grid rule, the adjudicator must rely on vocational expert testimony or other acceptable evidence to demonstrate the existence of other jobs. If a Grid rule directs a finding of disability or the evidence establishes that there is no other work available for an individual with the claimant's RFC and vocational profile, the claimant is found disabled. See Bowen v. Yuckert, 482 U.S. 137, 140-142 (1986).
D. Burden of Proof. A claimant for disability benefits bears the ultimate burden of proving that he is disabled within the meaning of the Social Security Act. 42 U.S.C. § 423(d)(5)(A); 20 C.F.R. § 404.1512. In terms of the sequential evaluation process described above, the claimant bears the burden of proving that he is not working, that he has a medically severe impairment or combination of impairments, and (unless he has demonstrated that his impairments are conclusively disabling under the Listings) that he is prevented by his impairments from performing his past work. Once such showings have been made, the burden of proof shifts to the Commissioner to demonstrate that the claimant can perform other work available in the national economy. See Bowen v. Yuckert, 482 U.S. 137, 146 n.5 (1987).
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