Stock's P/E Ratioby Thomas Smith
A stock's P/E ratio will fluctuate constantly depending on current earnings expectations and interest rates. It is very difficult to project what the P/E ratio will be at any given point in the future. A stock's historic average P/E ratio is probably as effective a predictor as any and better than most.
Source: http://www.investopedia.com/articles/stocks/09/steady-growth-stocks.asp; http://www.thomasmith.com/; http://www.testudoinvesting.com