What is the burden of establishing a disability by demonstrating that the claimant can perform other types of work in the national economy?

In disability cases, the claimant bears the initial burden of establishing his disability by demonstrating that he cannot return to his former job or former type of employment. The burden then shifts to the Secretary to show that claimant can perform other types of work in the national economy. The Secretary can satisfy this burden either by (1) applying medical vocational guidelines, "grids," or (2) taking testimony of vocational experts. Burkhart v. Bowen, 856 F.2d 1335, 1340 (9th Cir.1988).