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1.10 Completely randomized designs 21When considering how many replicates to use, it is useful to remember the formula forthe standard error for the difference between two means:s.e.d. = { (residual-mean-square) × (n 1 + n 2) / (n 1 × n 2) }Usually it will be appropriate to have the same replication for each treatment. The mainexception to this is that extra replicates are usually added for control treatments when themain interest is in comparing the other treatments with the control.We explain later how to use GenStat's design and randomization menus to assess howmany replicates are needed, and set up the design automatically.