当天下雨时，空气中就会比平时含有更少的氧气——比我们悲观估计的下降幅度更大。
When it is raining, one lungful of air will often have a few per cent less oxygen than on an average day - a larger reduction than in our pessimistic scenario above.
其中，EV=推导出的估计，O=乐观估计，L=最可能的估计和 P=悲观估计。
where EV=Derived Estimate, O=Optimistic, L=Most Likely, and P=Pessimistic.
要指出的是，在此情况下，对时间的估计悲观一些可能实际上非常重要。
One thing to point out is that pessimism can actually be very important in this situation.
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