When is one fund really a substitute for another?

Financial advisors have more investment choices than ever, but a larger fund universe creates a basic due-diligence problem: before deciding whether one fund is better than another, how do we know the two are actually comparable?

Fund categories are a useful starting point, but they can hide meaningful differences. Two funds may share a category while producing materially different return behavior. Conversely, funds with different labels, benchmarks or portfolio-construction approaches may behave similarly enough to represent realistic alternatives.

A more direct approach is to begin with realized returns. For an advisor considering whether to replace an existing ETF or mutual fund, the first question should be whether the proposed alternative has historically behaved like the current holding. Return similarity can help answer that question, while tracking error helps quantify the economic magnitude of the differences.

These measures serve a different purpose from performance rankings. A fund can outperform another fund while being a poor substitute for it. If their return paths are substantially different, the comparison may say more about taking different risk than about finding a better implementation of the same investment objective.

That distinction matters for client portfolios. Consider an advisor reviewing a long-held fund alongside a less familiar alternative that has generated stronger historical results. Looking only at excess return can make the alternative appear compelling. But if tracking error is large, replacing the original holding may materially change the portfolio's risk profile.

The due-diligence process therefore needs to separate two questions. First: Are these funds sufficiently comparable? Second: Conditional on being comparable, is there credible evidence that one has historically provided a better outcome?

This sequencing helps avoid a common selection problem: searching a large fund universe for the strongest performers and only afterward constructing a rationale for why they are substitutes. It also creates a more disciplined framework for interpreting historical results. Evidence of relative performance becomes more meaningful when it is observed between investments that have demonstrated similar behavior.

The same logic can improve ongoing monitoring. If a replacement candidate was selected because it closely tracked an existing holding while offering stronger evidence of relative performance, an advisor can continue to monitor both dimensions over time. A change in similarity may indicate that the funds no longer serve the same portfolio role, even if the relative-performance record remains attractive.

The final challenge is persistence. Historical similarity and relative performance are useful only to the extent that they contain information about future behavior. That makes out-of-sample testing particularly important. A fund-selection framework should be evaluated on relationships estimated using only information available at the time and then measured against subsequent outcomes.

None of this eliminates the traditional considerations involved in a fund replacement. Taxes, transaction costs, liquidity, fees, client objectives, account type and investment-policy constraints remain essential. A quantitative comparison should narrow the research set and improve the quality of the question, not replace professional judgment.

For advisors reviewing thousands of ETFs and mutual funds, the objective is not simply to find funds that performed better in the past. It is to identify genuinely comparable alternatives first - and then determine whether the evidence supports taking a closer look.

Vince (Qijun) Chen, CFA, is the founder of PortfolioFuture, an investment research platform focused on ETF and mutual fund alternatives, fund replication and empirical portfolio research. PortfolioFuture is a data-driven investment research platform for ETFs and mutual funds. Its research focuses on fund alternatives, return similarity, fund replication and out-of-sample validation.

Disclosure

The author is the founder of PortfolioFuture. The article discusses general fund due-diligence concepts and does not promote a specific investment product or security.