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Hello,You are mistaking two different things for the same : the number of samples in the experiment and the conduct of the experiment itself.The number of samples will determine the statistical power of the study. It depends on a lot of factors.The number of replicates you run for each measurement/ test/ experience... only determine if you will have a important variation in your technique. This is called the technical replicate, and it is done in triplicate because it is the minimum number to have a standard deviation. This way you ensure that your technique gives the same result each time you test the same sample.Only when you have satisfactory technical reproductibility will you look into the statistics of your experiments, and the number of samples required. The triplicate only ensure that you will be able to compare your different samples between them.
If you do it twice and get different answers you don't know which is right.If you do it 3 times and 2 agree you can still be reasonably sure that those 2 are right and the third went awry.However the number of replicates often depends on other factors- the cost and how important the outcome is are two common factors that affect the experiment.Experimental design is an important subject in its own right.http://en.wikipedia.org/wiki/Design_of_experiments