From Bob Herbert's very good column, "Education, Education, Education," a year ago in The New York Times: "Many have been left behind by the modest economic recovery of the past few years, especially those with limited education credentials." Per the rule below, we think Bob meant to write "educational," just as folks should more properly say "Democratic" when they refer to "Democrat" legislators.
In Harbrace Chapter 4, Adjectives and Adverbs, Rule 4d reads--
Avoid the awkward or ambiguous use of a noun form as an adjective.
Many noun forms effectively modify other nouns (as in reference manual, windfall profits tax, House Ways and Means Committee), especially when appropriate adjectives are not available. Avoid such forms, however, when they are awkward or confusing.
AWKWARD Many candidates entered the president race.
BETTER Many candidates entered the presidential race.
CONFUSING The Representative Landor recess maneuvers led to victory.
BETTER Representative Landor's maneuvers during the recess led to victory.