You can picture a line drawn forward from today: this is how far we expect it to run. The line is drawn from records of how far the same kind of thing has run before, so it is an estimate about a group, never a promise about one case. Whether the line belongs to a person or to a boiler changes nothing about how it is drawn.
可以想成從今天往前畫出去的一條線：我們預期它能走到這裡。這條線是根據同一類的東西以前走多遠畫出來的，所以它是對一群對象的估計，不是對單一個案的保證。這條線是屬於一個人，還是屬於一台熱水器，畫法完全一樣。
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