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I have a CRSP stock dataset that goes up to december 2013. I'm trying to append yahoo finance data to it in order to bring it up to the current day. However, there are ticker symbols that once belonged to a different company than they do today. For example, until 20121015 AAC was the ticker symbol for Australia Acquisition Corp, but now it's the ticker symbol for AAC holdings, a pharmaceuticals company. I don't have a source of permnos for the yahoo finance stocks, so that's not an option for me to keep track of which stock is which. Is there a minimum interval of time between when a stock symbol is no longer used by one company and begins to be used by another?

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    $\begingroup$ Just heuristically from having done this for a number of years, I haven't seen anything less than a year. (And these days there will often be a transition symbol which is helpful.) But it's not really tractable (or pleasant!) without using another identifier - CUSIP is readily available in CRSP, sedol is gonna be more friendly when you start to look at international instruments. $\endgroup$ – Gene Apr 28 '15 at 13:32

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