This is a very interesting post on All Things Lighting on the history and reason behind the Daylight Factor as well as a comparison of ten daylight simulation programs with a surprising result ..

Calculating daylight factors requires a 3D CAD model and daylight simulation software, if only to accurately model the spatial luminance distribution of the CIE Standard Overcast Sky and room surface interreflections. The obvious question is, how accurate are daylight simulation programs?

Given that these programs use a wide variety of radiosity and ray tracing techniques, you might hope to see at least reasonable agreement among their daylight factor predictions.
What the study revealed however was stunning: all but one of the programs agreed to within a few percent of each other.

Read the full post here ..