If I've got it, I'm assuming that you're still at school. Bearing this in mind, I'd advise you to think very carefully about pursuing this investigation. The final decision will remain yours, but I think you're teacher should have made you aware of the pitfalls.
Firstly, I'd like you to think on the way you're going to get the cohorts to consume the caffeine. If you're thinking of caffeine via instant coffee etc, your placebo group have to consume a liquid that tastes like coffee yet contains no caffeine. Now the obvious answer to this is decaffeinated coffee. However, the word decaffeinated is a misnomer as even that is not caffeine free. The caffeine is reduced in comparison to real coffee but not eliminated altogether. Now even a minute amount of caffeine in these circumstances can affect cognitive ability, so you cannot say that one cohort is consuming no caffeine whilst the other is doing so.
Let's move on to caffeine tablets. Now, you'd agree that someone has to give these tablets to the different groups. This means that you'd be doing what are known as either a single-blind or double-blind trial. I think you might find it useful if you looked up these terms on wikipedia etc for you to see if you think can conform to the thinking behind these trial types. These trial types are recognised scientific methods and varying them to fit your own needs is not really acceptable because your conclusions might be in error. In addition, if you're talking about handing each participant a caffeine tablet or placebo, you have to remember that the placebo must be absolutely identical in colour, size and weight to the caffeine tablet. How would you source such a placebo?
Returning to these single and double blind trials, do you consider that you wouldn't influence the cohort groups when you give out the real caffeine tablets and/or placebos?
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