Mid-cap core exchange-traded funds (ETFs) provide broad coverage of the U.S. mid-cap equities market. These funds are designed to serve as long-term core holdings in diversified ETF portfolios. The mid-cap category is generally understood to include companies with market capitalizations between $2 and $10 billion. However, none of the indexes that underlie the top ETFs in this space follow this simplified definition of the mid-cap range. As a result, the mid-cap core ETFs described below offer investors a good range of unique options for exposure to this area of the equities market.

1. Vanguard Mid-Cap ETF

The Vanguard Mid-Cap ETF (NYSEARCA: VO) delivers broad coverage of U.S. mid-cap equities by tracking the CRSP U.S. Mid Cap Index, a benchmark created and managed by the Center for Research in Security Prices at the University of Chicago Booth School of Business. This index ranks U.S. companies by market capitalization and then selects those companies ranked between the 85th and 70th percentiles. Some stocks near the edges of the selection band may be included in the index on a partial-weight basis. VO seeks to replicate the investment results of the index by investing in the same stocks in approximately the same proportions whenever possible.

As of December 2015, VO has net assets of more than $66.3 billion invested in 370 stocks. The largest holding in the fund, AvalonBay Communities, has a market capitalization of approximately $25.1 billion. The smallest holding in the fund, CONSOL Energy, Inc., has a market capitalization of $2 billion. The fund's median holding has a market cap of $10.9 billion. In terms of sector exposure, financial stocks lead the way with an allocation of 19%, followed by industrial stocks at 16.3%, consumer goods stocks at 14.4%, consumer services stocks at 14.1% and technology stocks at 12%. VO has a very low expense ratio of 0.09%.

2. IShares Core S&P Mid-Cap ETF

The iShares Core S&P Mid-Cap ETF (NYSEARCA: IJH) tracks the investment results of the S&P MidCap 400 Index, which includes U.S. companies with market capitalizations between $1.4 and $5.9 billion at the time of inclusion. Companies within this selection band are chosen for the index based on a variety of factors including adequate liquidity, general financial viability and industry classification. The index includes companies in a mix of industries to ensure sector representation is generally in line with the full population of eligible mid-cap companies. Since the index is not reconstituted on a regular basis, companies that rise or fall out of the initial selection band may remain in the index for some time.

IJH employs a representative sampling strategy to create a portfolio of stocks that closely approximates the characteristics of the underlying index. As of December 2015, IJH has approximately $26.5 billion in net assets across 400 stocks. The fund's top holding, Church & Dwight Company, has a market capitalization of $11.4 billion. The smallest holding in the fund, Talen Energy, has a market capitalization of $990 million. The median company in the fund has a market capitalization of about $3.4 billion. The sector breakdown includes a 26.5% allocation to financial stocks, 16.4% to information technology stocks, 15.4% to industrial stocks and 12.7% to consumer discretionary stocks. IJH has an expense ratio of 0.12%.

3. Schwab US Mid-Cap ETF

The Schwab US Mid-Cap ETF (NYSEARCA: SCHM) seeks to match the investment performance of the Dow Jones U.S. Mid-Cap Total Stock Market Index. This index is constructed by first ranking the companies in the Dow Jones U.S. Total Stock Market Index by market capitalization, and then selecting the group of companies ranked from 501 to 1000. Once in the index, companies are not removed unless they fall below a rank of 1100 or rise above a rank of 401. SCHM utilizes a sampling strategy to approximate the investment profile of the underlying index.

As of December 2015, SCHM has net assets of over $1.9 billion spread across 511 stocks. The top holding in the fund, Realty Income Corporation, has a market capitalization of approximately $12.4 billion, while the smallest, Inovalon Holdings, Inc., has a market capitalization of just under $2.8 billion. The median company in the fund has a market capitalization of $4.5 billion. In terms of sector allocation, 26.3% of the fund is invested in financial stocks, 16.2% in industrial stocks, 14.3% in consumer discretionary stocks, 14% in information technology stocks and 10% in health care stocks. SCHM has an exceptionally low expense ratio of 0.07%.

4. IShares Russell Mid-Cap ETF

The iShares Russell Mid-Cap ETF (NYSEARCA: IWR) offers investors broad exposure that tilts slightly more toward larger companies than other options on this list. IWR seeks to track the Russell Midcap Index. This index is comprised of the smallest 800 or so companies in the Russell 1000 Index, which itself includes the largest 1,000 companies in the United States. IWR employs a sampling strategy to track the underlying index.

As of December 2015, IWR has more than $12.8 billion in net assets invested in 831 holdings. The fund's largest holding, Southwest Airlines, has a market capitalization of approximately $30.6 billion. The smallest, EP Energy, has a market capitalization of $1.3 billion. The median company in the fund has a market cap of $5.6 billion. The sector breakdown includes a 23% allocation to financial stocks, 16.2% to consumer discretionary stocks, 15.3% to information technology stocks and 12.8% to industrial stocks. IWR has an expense ratio of 0.2%.

5. IShares Morningstar Mid-Cap ETF

The iShares Morningstar Mid-Cap ETF (NYSEARCA: JKG) attempts to track the investment results of the Morningstar Mid Core Index. To construct this index, Morningstar employs a proprietary method to select mid-cap companies characterized by average growth and value qualities. Under this methodology, mid-cap companies are those that rank between the 90th and 70th percentiles in terms of market capitalization in the U.S. Eligibility requires a listing on the New York Stock Exchange, NASDAQ or the NYSE Amex Equities market. Stocks in the index are weighted based on the numbers of shares publicly available for trading. JKG uses a representative sampling strategy to approximate the index.

As of December 2015, JKG has net assets of nearly $524 million in 213 stocks. Top holdings in the fund include Activision Blizzard, Inc. at 1.25%, Fidelity National Information Services at 1.19%, and Paychex, Inc. and Dr Pepper Snapple Group at 1.13%. Financial stocks are allocated at 21.1%, consumer discretionary stocks at 18.8%, information technology stocks at 18.6% and industrial stocks at 16.6%. JKG has an expense ratio of 0.25%.

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