While the P/E ratio is Wall Street's most common measurement for knowing whether a stock is cheap or expensive at the moment, it isn't always the best one. In this video, Motley Fool consumer-goods analyst Blake Bos looks at Sirius XM (SIRI) and tells investors why its normalized P/E ratio is much higher than it looks. And at that high multiple, Sirius is priced as a growth company, but with its moat eroding as Internet radio becomes more competitive in automobile ecosystems, it may not be the growth story it once was.