然后他测验，在从1871年到2009年的这段时期内，这样的策略产生了怎样的效果。
He then tested how such a strategy would have worked over the period from 1871 to 2009.
如果我们看一看一个时期以来的测验结果，我们常常可以发现一种趋势。
If we look at polls that are conducted over time, we can often detect a trend.
如果我们看一看一个时期以来的测验结果，我们常常可以发现一种趋势。
If we look at polls that are conducted over time, we can often detect a trend.
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