I was planning to use the particle board to provide a level surface and
didn't want to mess with leveling compound. The instructions
(http://www.quick-step.com/downloads/pdf/ENG.pdf) mention the Uniclic
Underfloor System. I purchased that but it wasn't clear to me from the
instructions whether it was for leveling or not. The instructions says
"a levelling underfloor must be installed", but I didn't think the
Uniclic Underfloor system was that because it just consisted of a thin
membrane. I'll double check tonight. I also was thinking of raising
the floor to the same level as the flooring in the adjoining bath and
hallway but if I can skip the particle board I certainly can live with
it as is.

Thanks for the response.