Mixing control areas with experimental areas
If variations between the areas are caused only by natural variations that always occur along beaches, then there is no reason for the averages to be different. When the experimental and control areas are chosen randomly, there should not be a larger supply of food only to the experimental area from natural causes. Since we have a mixture of experimental and control areas, every set of areas (experimental and control), should on average be equal because all of them have been exposed to the same natural variation.
If on the contrary it is as prescribed in the hypothesis, we expect on average an increase in the number of mussels within the experimental areas after competitors have been removed, independent of natural variations between the different places. To be able to make a proper comparison of these averages we must have several control and experimental areas.
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