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The full value of XP will not come until all the practices are in place. Many of the practices can be adopted piecemeal, but their effects will be multiplied when they are in place together.
Software programmers are used to dealing with the 20–80 rule—80% of the benefit comes from 20% of the work. XP makes use of this rule itself—put the most valuable 20% of functionality into production, do the most valuable 20% of the design, rely on the 20–80 rule to defer optimization.