Investors have enjoyed a strong rally all year long with very little in the way of pullbacks. There has been quite a bit of pressure lately from analysts all over Wall Street that say the markets are getting expensive and are due to pullback. Goldman Sachs (GS  ) even sent a letter to clients stating the "the markets are overpriced in every possible way." Not only that, but September is usually a weak month for stocks so let's take a look at some statistics to see what September has in store.

If we go back 20 years and measured the S&P 500's performance in the month of September we find that the average return is -0.75%. Now if that was all that happened over the month of September it would not likely scare many long term investors that are sitting on double digit gains this year alone.

Technology stocks, on the other hand show a much weaker history. Looking back over the same 20 year period we see that tech stocks tend to sell off an average of 1.66%. With the recent run up in the FANG names along with many other tech stocks, traders will likely be quick on the trigger to sell if it looks like history is about to repeat itself once again.

Lastly, are the material stocks which seem to perform the worst out of all the sectors. The 20 year average performance in September for the materials sector comes in at -2.72%. Unlike the tech stocks the materials sector has seen a steady and healthy uptrend, meaning it has had many up drafts as well as down drafts. Materials may be able to weather a selloff as this is the pattern its already following.

So there you have it, a little history to trade by. Long term investors will not likely make adjustments to such small moves overall, but the short term trader now has some information to at least be cautious of.