It was possible because economists are not obliged to make their data and programs publicly available when publishing scientific research. It is said that NBER working papers are even more prestigious than publication in refereed journals.
而之所以可能出现这种情况，是因为经济学家发表其科研成果时并不需要公开其数据和研究手法。据说发表在美国国家经济研究局的工作文件比发表在期刊上的更加权威，但该机构并未要求学者在其网站上公布其数据和研究程序。