Fischer Black used to call the stock market "efficient" because in his view it was almost always between 50% and 200% of fair value (he wasn't joking either; this was seriously his view and he nevertheless believed that the stock market was informative and regarded himself as an efficient markets/rational expectations believer). A range of 10-30 would be small compared to this and if one took the view, one wouldn't need a model of the fluctuation other than an anchor price around $20 plus "noise".
Depending on the probability distribution (fat tails?) and speed of adjustment he had in mind, swings in the value of social wealth within his range could be pretty wild. Considering the welfare loses that would result, Black's "efficient" stock market could be terribly inefficient from an economic point of view (economic efficiency = maximal welfare). This sounds to me like a very nonchalant view of market efficiency.
Julio
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