The move lower in stocks this week naturally has investors wondering if this is just a speed bump in the really or the start of a more serious correction.

Exchange traded funds (ETFs) for the major stock indexes have risen the first four months of the year — iShares S&P 500 (NYSEArca: IVV) is up about 8% year to date with a slight drop-off seen in recent days.

Should investors sell in May and go away? One year after the May 6 flash crash, investors know this is when stocks fell in 2010.

However, some analysts think the S&P 500 does have room to run up for the rest of this year. Russ Koesterich, iShares Global Chief Strategist, says the aspects to pay attention to are margins and multiples.