We can all agree on the substance of that statement--eight percent of eighth graders doing math at grade level is criminal.
Whether or not we'd ever literally agree with that, figuratively it is still dependent on a number of factors, including how do you define grade level?
In this case, the 8% figure is based on the level at proficient or above accorting to NAEP.
But, as Gerald Bracey wrote:
The National Academy of Sciences put it this way: “NAEP’s current achievement-setting procedures remain fundamentally flawed. The judgment tasks are difficult and confusing; raters’ judgments of different item types are internally inconsistent; appropriate validity evidence for the cut scores is lacking; and the process has produced unreasonable results.”
The academy recommended use of the levels on a “developmental” basis (whatever that means) until something better could be developed. In 1996, the National Academy of Education recommended the current achievement levels “be abandoned by the end of the century and replaced by new standards … .”
Thanks to GFBrandenburg for a little help with this.