Looking today at week-over-week shares outstanding changes among the universe of ETFs covered at ETF Channel, one standout is the SPDR S&P MidCap 400 ETF (MDY) where we have detected an approximate $838.2 million dollar inflow -- that's a 5.4% increase week over week in outstanding units (from 65,728,538 to 69,303,538). Among the largest underlying components of MDY, in trading today Alliance Data Systems Corp. (ADS) is trading flat, Affiliated Managers Group Inc. (AMG) is up about 0.1%, and LKQ Corp (LKQ) is lower by about 0.1%. For a complete list of holdings, visit the MDY Holdings page »The chart below shows the one year price performance of MDY, versus its 200 day moving average: Looking at the chart above, MDY's low point in its 52 week range is $171.53 per share, with $236.95 as the 52 week high point — that compares with a last trade of $234.26. Comparing the most recent share price to the 200 day moving average can also be a useful technical analysis technique -- learn more about the 200 day moving average ». Exchange traded funds (ETFs) trade just like stocks, but instead of ''shares'' investors are actually buying and selling ''units''. These ''units'' can be traded back and forth just like stocks, but can also be created or destroyed to accommodate investor demand. Each week we monitor the week-over-week change in shares outstanding data, to keep a lookout for those ETFs experiencing notable inflows (many new units created) or outflows (many old units destroyed). Creation of new units will mean the underlying holdings of the ETF need to be purchased, while destruction of units involves selling underlying holdings, so large flows can also impact the individual components held within ETFs.