S000001273 [Member] Investment Risks - MID CAP INDEX FUND

Mar. 31, 2026

TRACKING RISK is the risk that the Fund’s performance may vary from the performance of the index it tracks as a result of share purchases and redemptions, transaction costs, expenses and other factors. Market disruptions, regulatory restrictions or other abnormal market conditions could have an adverse effect on the Fund’s ability to adjust its exposure to required levels in order to track its Index or cause delays in the Index’s rebalancing schedule. During any such delay, it is possible that the Index, and, in turn, the Fund will deviate from the Index’s stated methodology and therefore experience returns different than those that would have been achieved under a normal rebalancing or reconstitution schedule.

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SECTOR RISK is the risk that companies in similar businesses may be similarly affected by particular economic or market events, which may, in certain circumstances, cause the value of securities of all companies in a particular sector of the market to decrease.

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INDUSTRIALS SECTOR RISK is the risk that companies in the industrials sector may be significantly affected by, among other things, worldwide economic growth, supply and demand for specific products and services, rapid technological developments, international political and economic developments, environmental issues, and tax and governmental regulatory policies.

MID CAP STOCK RISK [Member]

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MID CAP STOCK RISK is the risk that stocks of mid‑sized companies may be more volatile than stocks of larger, more established companies, and may lack sufficient market liquidity. Mid‑sized companies may have limited product lines or financial resources, may be dependent upon a particular niche of the market, or may be dependent upon a small or inexperienced management group. Securities of mid‑sized companies may trade less frequently and in lower volume than the securities of larger companies, which could lead to higher transaction costs. Generally, the smaller the company size, the greater the risk.

INDEX RISK [Member]

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INDEX RISK is the risk that that the Fund would not necessarily buy or sell a security unless that security is added or removed, respectively, from the Index, even if that security generally is underperforming, because unlike many investment companies, the Fund does not utilize an investing strategy that seeks returns in excess of the Index. Additionally, the Fund rebalances its portfolio in accordance with the Index, and, therefore, any changes to the Index’s rebalance schedule will result in corresponding changes to the Fund’s rebalance schedule.

DERIVATIVES RISKS [Member]

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DERIVATIVES RISK is the risk that derivatives may pose risks in addition to and greater than those associated with investing directly in securities, currencies and other instruments, may be illiquid or less liquid, more volatile, more difficult to value and leveraged so that small changes in the value of the underlying instrument may produce disproportionate losses to the Fund. Derivatives are also subject to counterparty risk, which is the risk that the other party to the transaction will not perform its contractual obligations. The use of derivatives is a highly specialized activity that involves investment techniques and risks different from those associated with investments in more traditional securities and instruments.

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FUTURES CONTRACTS RISK is the risk that there will be imperfect correlation between the change in market value of the Fund’s securities and the price of futures contracts, which may result in the strategy not working as intended; the possible inability of the Fund to sell or close out a futures contract at the desired time or price; losses due to unanticipated market movements, which potentially are unlimited; and the possible inability of the Fund’s investment adviser to correctly predict the direction of securities’ prices, interest rates, currency exchange rates and other economic factors, which may make the Fund’s returns more volatile or increase the risk of loss.

SECURITIES LENDING RISK [Member]

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SECURITIES LENDING RISK is the risk that the Fund may lose money because the borrower of the loaned securities fails to return the securities in a timely manner or at all. The Fund could also lose money in the event of a decline in the value of collateral provided for loaned securities or a decline in the value of any investments made with cash collateral.

MARKET RISKS [Member]

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MARKET RISK is the risk that the value of the Fund’s investments may increase or decrease in response to expected, real or perceived economic, political or financial events in the U.S. or global markets. The frequency and magnitude of such changes in value cannot be predicted. Certain securities and other investments held by the Fund may experience increased volatility, illiquidity, or other potentially adverse effects in response to changing market conditions, inflation, elevated levels of government debt, changes in interest rates, lack of liquidity in the bond or equity markets or volatility in the equity markets. Market disruptions caused by local or regional events such as financial institution failures, changes in trade regulation or economic sanctions, internal unrest and discord, war, acts of terrorism, the spread of infectious illness (including epidemics and pandemics) or other public health issues, recessions, the threat or occurrence of a government shutdown, or other events or adverse investor sentiment could have a significant impact on the Fund and its investments. During periods of market disruption or other abnormal market conditions, the Fund’s exposure to risks described elsewhere in this summary will likely increase.

Risk Lose Money [Member]

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As with any mutual fund, it is possible to lose money on an investment in the Fund.

Risk Not Insured Depository Institution [Member]

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An investment in the Fund is not a deposit of any bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation, any other government agency, or The Northern Trust Company, its affiliates, subsidiaries or any other bank.

Risk Nondiversified Status [Member]

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NON‑DIVERSIFICATION RISK Under the 1940 Act, a fund designated as “diversified” must limit its holdings such that the securities of issuers which individually represent more than 5% of its total assets must in the aggregate represent less than 25% of its total assets. The Fund is “diversified” for purposes of the 1940 Act. However, in seeking to track its Index, the Fund may become “non‑diversified,” as defined in the 1940 Act, solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the Index. A non‑diversified fund can invest a greater portion of its assets in the obligations or securities of a small portion of its assets in the obligations or securities of a small number of issuers or any single issuer than a diversified fund can. In such circumstances, a change in the value of one or a few issuers’ securities will therefore affect the value of the Fund more than if it was a diversified fund.