Comparison between Nikkor and Sigma lens?

Can anyone provide me links comparing Nikkor 18-200mm and 18-55mm lens to Sigma counterparts?

I don't want any forums or what people think but link to some tests if they have been done.

DPreview tested both the Nikkor 18-200mm and its Sigma counterpart on the D300. The interface is a bit clunky, but you get the hang of it:

http://www.dpreview.com/...on.xml%3F3

All four lenses were tested at Photozone.de, in this case, with Canon mount Sigmas. If you normalize for the difference in nyquist frequencies between the camera bodies used for testing, you can ignore the prominent disclaimer and compare them tit-for-tat:

Nikkor 18-200mm: http://www.photozone.de/...est-report

Nikkor 18-55mm: http://www.photozone.de/...est-report

Sigma 18-200mm: http://www.photozone.de/...rt--review

Sigma 18-50mm: http://www.photozone.de/...rt--review

SLRgear.com also tested both of the superzooms, again with a Canon mount Sigma. You can compare the results here:

Nikkor 18-200mm: http://www.slrgear.com/...250/cat/13

Sigma 18-200mm: http://www.slrgear.com/...007/cat/31