Studies about pronunciation guides

On page 121 of Landau's 2001 2nd ed of /Dictionaries: The art and craft of lexicography/, he states that there is a movement to make pronunciation guides simpler than the ones using diacritics (as in use in American dictionaries), because "studies have shown that the respelling systems currently in use are widely misunderstood."
Does anyone have any idea what studies might be meant? (other than the ones by Fraser in 1996 and 1997, which didn't really test the American-style systems at all) (I've searched in various research indices but have come up empty-handed.)
thanks!
Carolyn