Wouldn't it be easier to isolate the other way - decaf coffee?
Also, tea? If I was listing "main caffeine delivery methods" I'm not sure tea would make the top 3 (probably top 5). Actually, let's see:
1. Coffee, no doubt
2. Energy drinks
3. Sodas (esp the citrus ones, but also inc. Dr. Pepper)
4. Caffeine pills
5. tea
Yeah I'm not sure about the relative ordering of 3 through 5. Might depend on how serious you are (I use sodas to soften the crash, not to get me going).