I'D SAY THIS WAS SLOPPY RESEARCH

Sam Pizzigati wrote on Campaign for America's Future site that the Federal Reserve doesn't include the Forbes 400 in its survey of American wealth.  I found that really hard to believe because of the vast wealth these 400 people have so I followed Pizzigati's link to the FRB report and found that he was correct:

The Sampling Techniques

The survey is expected to provide a core set of data on family income, assets, and liabilities. The major aspects of the sample design that address this requirement have been constant since 1989. The SCF combines two techniques for random sampling. First, a standard multistage area-probability sample (a geographically based random sample) is selected to provide good coverage of characteristics, such as homeownership, that are broadly distributed in the population.

Second, a supplemental sample is selected to disproportionately include wealthy families, which hold a relatively large share of such thinly held assets as noncorporate businesses and tax-exempt bonds. Called the "list sample," this group is drawn from a list of statistical records derived from tax returns. These records are used under strict rules governing confidentiality, the rights of potential respondents to refuse participation in the survey, and the types of information that can be made available. Persons listed by Forbesmagazine as being among the wealthiest 400 people in the United States are excluded from sampling.

Of the 6,492 interviews completed for the 2010 SCF, 5,012 were from the area-probability sample, and 1,480 were from the list sample; for 2007, 2,914 were from the area-probability sample, and 1,507 were from the list sample. The number of families represented in the surveys considered in this article is given by the following table: