September 2013
Intermediate to advanced
264 pages
5h
Chinese
Content preview from Bad Data 技術手冊
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第九章：當資料與現實不相符時
簡單來說，若你現在在看 Sears Holdings，並且想知道在 2004 年它的股票編號為何，答
案是 KMRT，並不是 S。若想知道它在 2001 年的股票編號，答案則是它尚未存在。但
若你想要查股票編號為 S 的過去某個時間點的價錢，那麼你最好確定你是在查 Sears 還
是 Sprint。
多數的人在工作上會使用股票的資料，就算是在財務企業中，也會想要使用股票編號當
成唯一辨識碼。它們當初被建立時就是基於這個目的，而且在任何時間點內都會是唯一
的。但當你開始建立時間序列來從過去看這些資料，它們將會是你的絆腳石。
多數的資料來源，如 Google 或是 Yahoo! Finance，都是使用最近的股票編號來當成唯一
鍵。這代表不論在何時讀取了更新資料，有些辨識碼已經變更了。
更重要的是，任何目前再也不能夠被交易的公司並沒有擁有一個編號。在目前多數的資
料集合中，若你尋找「S」則會找到 Sprint；若尋找「Sears」則會找到 SHLD；若尋找
Kmart 則什麼都找不到。這在資料中導致了一個主要的「存活偏差」：若你從不去看那
些破產倒閉或是被買下的公司，該股票市場看起來會非常的有利。
也有一些縮寫為 CUSIP、ISIN 或是 SEDOL 等的通用唯一辨識碼。但這些是私營的並且
通常是無法取用的。它們也只有解決一半的問題：它們不會像股票編號般被回收使用，
但它們會因為該股票有小變動而變更它們的辨識碼（如當 KMRT 變成 SHLD 時 CUSIP
就變更了） ...
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