Multilevel switching and personal dimming are often lumped together because the goal is the same–providing users a manual choice of light levels. The 2003 Advanced Lighting Guidelines, published by the New Buildings Institute, says they both save about 30% in energy.
There is evidence that bi-level switching saves much more energy than personal dimming control, but personal dimming control is more comfortable and satisfying to users.
And when I say bi-level switching, I mean multi-level switching, where the inboard and outboard lamps in 3-lamp fixtures are separately circuited and independently controlled via switches. This allows there light and power levels, not two, specifically 33%, 66% and 100% besides OFF.
In previous research, it demonstrated energy savings of 8-22% (8% classrooms, 16% open offices, 22% private offices, ADM Associates, 2002) and 24% (San Francisco Federal Building project/study).
Personal dimming control has demonstrated energy savings of 6% (National Center for Atmospheric Research or NCAR study) and 11% (National Research Council Canada or NRC study, 2007). The savings are around 10%.
I think people are happier with personal dimming control and tend to stick with light levels they’ve chosen, which are retained in memory. But the energy savings just aren’t as high as when people are forced every time they enter their workspace to make a choice between stark lighting increments that also produce higher savings increments.