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What is the essential purpose of blocking, and how should the blocks be designated?
Blocking reduces variability within the blocks; individuals within each block should be as similar as possible.
Blocking reduces measurement error; individuals should be self-selected in each block.
Blocking makes subjects more comfortable; subjects select their own groups.
Blocking increases the sample size, which in turn reduces variability; individuals within blocks should be as different as possible to create a more heterogeneous experiment.
Blocking reduces bias in experiments; individuals should be randomly divided into their blocks.