Posted 09 May 2006 - 06:45 AM
It's common sense, really.
Think of linking to other businesses like allowing another business to leave their business cards on your checkout desk, and they put your cards on their desk.
Do you want people to think your business is somehow related to or endorsing the other business? Because that's what you are doing. A link says "We recommend this site."
If you have a reason to recommend it, (ie, you think it's a cool site, it belongs to a friend, they have the best prices on accessories your clients will want, it's in your local area, etc, etc) then link to it!
It's really that easy.
Do I want to recommend this site (and do I want them to recommend mine?) If yes, then link away. If it gives you an uneasy feeling or you can't think of a reason why you'd want to link to them (other than trying to manipulate your link popularity), don't do it.
You don't need tests or numbers. I can't imagine how you would test that anyway since there's no way to create a "pure test environment". Other factors will always be skewing your results and it's easy to think you've "proved" something but very difficult to reproduce it. If you can't consistently get the same results, you can't prove that x caused y.
Common sense is the way to go!