Indexes · Mid cap
The S&P 400 Mid-Cap Band
The only band in the family with a boundary at both ends. Its members leave by growing and by shrinking, and the index trades at both edges, which is the one thing a middle band has that the top and bottom do not.
Where it sits
Three size bands share one methodology and one committee: the large-cap 500, this mid-cap 400, and the small-cap 600. The eligibility rules are identical throughout (American domicile, an eligible listing, sufficient float and liquidity, and a positive-earnings screen), which is what makes the three comparable with each other and not with the Russell family, whose indices are rank-based and apply no earnings test.
Migration is the distinctive feature
Every index has turnover. What a middle band has is turnover from two directions, and it is worth following through because it is the mechanism behind the largest volume events most mid-cap companies ever see.
Promotion. A company grows enough to be moved up. Funds tracking this index sell it; funds tracking the band above buy it. Both trades happen on the same date at the same reference price, and both are known weeks in advance.
Demotion. A company shrinks and is moved down. The same mechanism runs in reverse, with this index selling and the small-cap index buying, and the demoted company is usually a business in difficulty, so the trade arrives in a market already unsettled about it.
Deletion. Acquisition, a failed eligibility test, or a corporate reorganisation. The index sells and nothing buys on the index’s behalf, so the flow is one-sided.
What the migration does to a volume reading
| Stage | What happens | Effect on the measures here |
|---|---|---|
| Announcement | The change is published, typically some days before it takes effect. | Volume rises immediately as participants position for the mechanical trade. A relative-volume reading here is real and has an identifiable cause. |
| Effective date | Every tracking fund trades at the reference price, usually in the closing auction. | The largest single-session volume figure most mid-caps ever record, frequently many multiples of normal. Not a market opinion. |
| The following weeks | Ordinary trading resumes at a level that may be permanently different, since the holder base has changed. | The event session sits inside every twenty-session baseline for a month, raising the denominator and making the weeks after it read as quiet. |
| Long run | A larger share of the register is held by index funds that do not trade on news. | Float available to active traders falls, which affects turnover comparisons and can raise the price impact of a given order. |
The third row is the one that catches readers of this site, and it is the same window problem the average volume page sets out, except that it bites harder here. A mid-cap’s ordinary volume is smaller, so an index event is a larger multiple of it, and the distortion to any baseline containing that session is correspondingly greater.
Why the earnings screen matters most in the middle
The positive-earnings requirement is the main methodological difference between this family and the rank-based alternatives, and its effect grows as company size falls. Among the largest companies almost everything qualifies; in the mid-cap range a meaningful number of businesses do not.
So an S&P mid-cap index and a rank-based mid-cap index hold materially different companies while claiming to describe the same segment of the market. Comparing their returns is comparing two selection rules, and attributing the difference to anything else (a manager’s skill, a factor, a market regime) is a mistake that is easy to make and easy to avoid. The small-cap page works through the same screen at the size where it excludes most.
What mid-cap exposure actually is
Two properties distinguish the band, and both follow from size rather than from anything intrinsic to the segment.
Lower liquidity. Positions take longer to build and to exit, index events move prices further, and the volume measures in the library behave less like their descriptions assume, the caution that appears in every "where it misleads" section applies more strongly here than to a large-cap universe.
More domestic revenue. Mid-sized companies earn a larger share of their revenue at home on average, which makes the band more sensitive to domestic conditions and less to currency moves than a large-cap index whose members are multinational. That is the substantive reason the two bands can diverge for quarters at a time, and it is a statement about where the companies do business rather than about size preference.
Neither property makes mid-caps a better or worse investment, and this reference takes no position on that. What they do is make the band a different exposure with different data properties, which is the thing worth knowing before a measure calibrated on large-cap data is applied to it.
Frequently asked questions
What is the S&P 400?
The mid-cap band of the S&P family: four hundred American companies sitting below the large-cap 500 and above the small-cap 600, selected by the same committee under the same eligibility rules, domicile, listing, float, liquidity and a positive-earnings screen. Together with those two indices it forms the Composite 1500, and the shared methodology is what makes the three comparable with one another and not with the Russell family.
What makes the middle band different?
It has a boundary at both ends. A company can leave the large-cap index only by falling out of eligibility or by being acquired; a company can leave the small-cap index by growing. The mid-cap band loses members in both directions — promoted upward when they grow and demoted downward when they shrink — which means it is the one band whose membership is disturbed from above and below at once.
What happens when a company is promoted?
Every fund tracking this index has to sell it and every fund tracking the band above has to buy it, on the same date at the same reference price. The trade is mechanical and known in advance, which is what makes index events the largest single-session volume figures most companies ever record, and why an index change is one of the four things that decide a monthly volume ranking.
Is the promotion good news for a holder?
Not in the way it is usually presented. The demand from the larger index is genuine and the supply from this one is equally genuine, and both are known weeks ahead, so the effect is largely priced before the date. Long-run studies of index-inclusion effects have found the initial jump reversing over subsequent months in many cases. Being promoted is a statement about how much the company has already grown.
Why does the earnings screen matter here?
Because it is the main difference between this family and the Russell indices at the same size range, and it bites hardest in the middle and at the bottom. Requiring positive reported earnings excludes companies that a purely rank-based index would include, so an S&P mid-cap index and a Russell mid-cap index hold different companies while claiming to describe the same segment. Neither is wrong; comparing their returns without noting the screen is.
How do mid-caps behave differently from large caps?
Two properties are consistent and both come from size rather than from anything mystical about the segment. Liquidity is lower, so a position takes longer to build and index events move prices further. And the constituents are less internationally diversified on average, which makes the band more sensitive to domestic conditions than a large-cap index whose members earn much of their revenue abroad. Neither property makes the band better or worse; they make it a different exposure.
What does this mean for the volume measures on this site?
That a mid-cap instrument’s volume baseline is more easily distorted than a large-cap one’s. A single index event, a placement or a results day is a larger multiple of ordinary volume when ordinary volume is smaller, so an event session sits inside a twenty-session average and raises it materially for a month. Every relative-volume reading in that window then understates how busy the instrument is, which is the window problem in its most pronounced form.
Where do I get the constituent list?
From S&P Dow Jones Indices, or from the daily holdings of any fund tracking the index. It is not reproduced here for the reason that applies to every index page in this section: membership changes as events require, a static table becomes wrong quietly, and a current list applied to historical data silently removes every company that was demoted or acquired: which in a middle band is a substantial number.