1. 本益比是報酬率的導數，計算上就是Price / Earning（簡稱P/E），通常財經媒體使用的P就是現在股價，但E卻是用去年每股淨利，這絕對是錯誤的！
因此，要正確地使用「本益比」這個工具，你需要的並不是拿「過去盈餘」計算，而是要拿「未來盈餘」才對，也才符合經濟學邏輯。明眼人都看出來，未來盈餘才是大麻煩與功夫所在，因為預測未來盈餘絕非易事，之中存在非常鉅額的「訊息成本(information costs）」。而要提高預測未來盈餘正確性，根據我的經驗，一流的economic sense不可少。
共同錯誤在於這些人都忽略的，本益比既然是報酬率的倒數，真正有意義的比較對象應該是機會成本 — 也就是放棄的最高代價。如前述，巴菲特選擇美國國債利率作為機會成本來比較，這就是為什麼2017/2/27接受CNBC訪問時，會說現在美股沒有泡沫化，同時此時美股不算貴：
Quick: Although you have had times where you thought stocks were incredibly cheap, like in 2008, 2009, when you talked about that, even on our program. You thought that there were times that stocks were greatly overvalued where you’ve said, “Forget it, don’t do it." Are we near an inflection point right now, as best as you can tell?
Buffett: Well … I’ve been talking this way for quite a while, ever since the fall of 2008. I was a little early on that actually. But I don’t think you could time it. And we are not in a bubble territory or anything of the sort. Now, if interest rates were 7 or 8 percent then these prices would look exceptionally high. But you have to measure, you know, you measure everything against: interest rates, basically, and interest rates act like gravity on valuation. So when interest rates were 15 percent in 1982 they’d pull down the value of any asset. So, what’s the sense of buying a farm on a 4 percent yield basis if you can get 15 percent in government’s? But measured against interest rates, stocks actually are on the cheap side compared to historic valuations. But the risk always is, is that — that interest rates go up a lot, and that brings stocks down. But I would say this, if the ten-year stays at 230, and they would stay there for ten years, you would regret very much not having bought stocks now.
Quick: Yeah, there are times, Warren, where you hear pundits or other people saying, “Look things are at risk at this point. Our American way of life, our system is under threat." And I’ve heard this from all sides at all different times. Is there ever a point where you thought that was the case?
Buffett: No, and you say you’ve heard it at all times from all sides. I’ve been hearing it, you know, all my life. And in the spring of 1942 I was 11 years old, and the Dow was at about 100. And we were losing the war in the Pacific at that point, that was early … was shortly after Pearl Harbor. And there was no doubt in this country we were going to win over time. I mean, and people said, “Well, this is let’s wait till things are clear, let’s wait till we start winning the war." There’s always a reason to wait and I’ve listened to that all my life. You know, when I got out of school the Dow had never been above 200. There’d never been a year when the Dow had not been below 200 during the year. Even in 1929, when it got to 381, the low was below 200. Never been a year. Well, so what, you know? But that was a big subject at that time. And then you know, we ran into price controls, we ran into the oil shocks, you name it, just all kinds of things. And those are diversions. So all my life I’ve been hearing, “You know, maybe there’s a better time to invest, you know?" Or, “Things are more unpredictable now." They’re always unpredictable. I can’t predict what’s gonna happen tomorrow. I mean, you could have anything happen tomorrow. We’ve had October 19th, 1987, 22 percent down in one day. So I can predict what’ll happen ten or 20 years in a general way, but I have no idea what’ll happen tomorrow. And the important thing is if you got these wonderful assets out there, to own ’em, and which ones do you own? I mean, if you … if you save money you can buy bonds, you can buy a farm, you can buy an apartment, house, or even buy a part of American business. And if you buy a 10-year bond now you’re paying over 40 times earnings for something whose earnings can’t grow. And you know, you compare that to buying equities, good businesses, I don’t think there’s any comparison. But that doesn’t mean the stock market can’t go down 20 percent tomorrow. I mean, you never know what it’s going to do tomorrow, but you do know what it’s going to do over ten or 20 years. And people talk about 20,000 being high. Well, I remember when it hit 200 and that was supposedly high. The Dow, I mean, the Dow, in your lifetime. You know, you’re going to see a Dow that certainly approaches 100,000 and that doesn’t require any miracles, that just requires the American system continuing to function pretty much as it has.
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他明明是說：「…you’re going to see a Dow that certainly approaches 100,000 and that doesn’t require any miracles, that just requires the American system continuing to function pretty much as it has.」