What is the main idea in this ruling? how could it impact united states v. fields?

This case involves a federal death sentence imposed on defendant-appellant Fields for conviction of a federal capital offense. Fields was sentenced to death largely on the basis of the opinion of a psychiatrist who stated that he could confidently predict Fields would be dangerous in the future. The psychiatrist testified that he did not know of any "standard psychiatric or medical procedures used in arriving at a determination or predicting future dangerousness" and that he was unaware of specific empirical data or studies. He issued his opinion without engaging in any testing or any other objective measures or use of an actuarial method. His basis for this opinion was discussions with the prosecutors and review of some records regarding the defendant. The defense attorney objected to the testimony as unreliable under the standards for expert testimony established by the U.S. 

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