When people talk about “the U.S. stock market,” they often mean big, famous companies like Apple, Microsoft, or Amazon. But those giants are only part of the picture. Beneath the headlines sits a much broader and often overlooked segment of the market—thousands of mid-cap and small-cap companies that quietly drive innovation and growth.
That is exactly where the Dow Jones U.S. Completion Total Stock Market Index comes in.
This index plays a critical role for investors who want true, full-market exposure. Yet it’s rarely explained in plain English. Let’s change that.
What Is the Dow Jones U.S. Completion Total Stock Market Index?
The Dow Jones U.S. Completion Total Stock Market Index (often shortened to Completion Index) tracks all U.S. stocks except those in the S&P 500.
In simple terms:
- S&P 500 = Large, established U.S. companies
- Completion Index = Everything else that’s publicly traded and investable
Together, the S&P 500 plus the Completion Index represent nearly the entire U.S. stock market.
This makes the Completion Index a powerful building block for diversified portfolios.
Why the Index Exists (and Why It Matters)
The S&P 500 covers roughly 80% of total U.S. market capitalization, but it includes only large-cap stocks. That leaves thousands of smaller companies out of the picture.
The Completion Index was created to fill that gap.
It includes:
- Mid-cap stocks (established but still growing companies)
- Small-cap stocks (younger, faster-growing firms)
- Micro-cap stocks (early-stage public companies)
These businesses are often:
- More growth-oriented
- More sensitive to economic cycles
- More volatile—but potentially more rewarding
Historically, small- and mid-cap stocks have delivered higher long-term returns than large caps, though with greater short-term risk.
How the Completion Index Is Structured
The index is market-capitalization weighted, meaning larger companies within the index carry more influence than smaller ones.
Key characteristics:
- Includes thousands of stocks
- Covers multiple sectors, including technology, healthcare, industrials, and consumer services
- Rebalanced periodically to reflect market changes
When a company grows large enough to enter the S&P 500, it is removed from the Completion Index. When companies fall out of the S&P 500, they may be added back.
This constant adjustment keeps the index accurate and relevant.
How Investors Actually Use the Completion Index
Most individual investors don’t buy the index directly. Instead, they access it through index funds or ETFs.
A common strategy looks like this:
- 80% S&P 500 fund
- 20% Completion Index fund
This combination closely replicates the total U.S. stock market, similar to what a total market index fund provides.
Institutional investors—such as pension funds and endowments—use this approach to fine-tune exposure without over-relying on mega-cap stocks.
Performance: Risk, Reward, and Reality
The Completion Index tends to be more volatile than the S&P 500.
Why?
- Smaller companies are more affected by interest rates
- Earnings are less stable
- Access to capital is more limited
However, over long periods, small- and mid-cap stocks have often outperformed large caps during:
- Economic recoveries
- Early expansion phases
- Innovation-driven cycles
For example, following major market downturns, smaller companies have historically rebounded faster as economic activity picks up.
This makes the Completion Index particularly attractive for long-term investors who can tolerate short-term ups and downs.
Completion Index vs Total Market Index
It’s easy to confuse these two, but the difference is important.
- Total Market Index: Includes everything (S&P 500 + Completion Index)
- Completion Index: Includes everything except the S&P 500
Think of the Completion Index as the missing puzzle piece rather than the full picture.
If you already own an S&P 500 fund, adding a Completion Index fund increases diversification without doubling up on large-cap exposure.
Who Should Consider Investing in the Completion Index?
This index is especially suitable for:
- Investors who already hold S&P 500 funds
- Long-term investors seeking higher growth potential
- Those building diversified, low-cost portfolios
- Investors comfortable with short-term volatility
It may be less suitable for:
- Short-term traders
- Income-focused investors
- Those uncomfortable with market swings
As always, asset allocation matters more than chasing returns.
Expert Perspective: Why Diversification Still Wins
Many investment professionals argue that portfolios overly concentrated in large-cap stocks are vulnerable to valuation risk.
With a small group of mega-cap companies dominating headlines and indexes, the Completion Index offers exposure to the broader economic engine of the U.S.—entrepreneurs, niche leaders, and emerging innovators.
In other words, it spreads risk where future growth may actually come from.
Final Thoughts: The Quiet Power of the Completion Index
The Dow Jones U.S. Completion Total Stock Market Index doesn’t grab attention like the Dow or the S&P 500. But it plays a crucial role behind the scenes.
It represents:
- The depth of the U.S. economy
- The growth stage of tomorrow’s market leaders
- A smarter way to achieve full diversification
For investors who want more than just the biggest names, the Completion Index completes the story—exactly as its name suggests.