Easy data bias explained

In this episode, we are joined by Elroy Dimson, Professor of Finance at Cambridge Judge Business School and co-creator of the Dimson-Marsh-Staunton (DMS) dataset, for a sweeping and deeply insightful conversation on financial history, market behavior, and the evolution of global investing. Elroy walks us through the origins of the groundbreaking Triumph of the Optimists, the challenges of assembling over 100 years of global return data, and the critical biases that once shaped our understanding of markets. We explore how expanding beyond U.S.-centric data reshaped expectations for the equity risk premium, why economic growth doesn't necessarily translate into higher stock returns, and what history reveals about diversification, factor investing, and investor behavior. Elroy also shares lessons from his work with major institutions like Norway's sovereign wealth fund, discusses the surprising long-term outperformance of railways, and offers a grounded perspective on future expected returns. This episode is a masterclass in using history to inform better financial decisions.

Key Points From This Episode:

(0:04:00) Introduction to Elroy Dimson and the significance of the DMS dataset.

(0:05:07) Why understanding financial history is essential for thinking about the future.

(0:05:24) The origin story of Triumph of the Optimists and assembling global return data.

(0:09:06) How long-term datasets are built from academic and commercial sources.

(0:11:33) Survivorship bias in historical indices and why it matters.

(0:13:35) "Easy data bias" and how it leads to overstated historical returns.

(0:15:32) Accounting for failed markets and geopolitical disruptions in global data.

(0:18:33) How global data changed expectations for the equity risk premium.

(0:21:09) Why 20th-century equity returns were a "pleasant surprise."

(0:22:17) U.S. market dominance and the challenge of extrapolating its success.

(0:24:11) Market composition in 1900 and the dominance of railway stocks.

(0:25:52) Why railways outperformed despite shrinking market share.

(0:29:03) The surprising disconnect between economic growth and stock returns.

(0:31:28) Why investing in recovering markets requires extreme patience and conviction.

(0:33:32) Value investing: historical success and recent struggles.

(0:35:00) Why economic growth benefits many—but not necessarily stock investors.

(0:35:59) The long-term benefits of global diversification.

(0:40:01) Why diversification reduces risk—but doesn't create returns for everyone.

(0:42:29) Explaining persistent home country bias among investors.

(0:47:46) Industry diversification becoming more important over time.

(0:49:50) The rise and evolution of size, value, and momentum factors.

(0:54:17) Why factor premiums should be monitored—not blindly followed.

(0:57:27) The equity risk premium: why it's crucial—and uncertain.

(1:00:15) A realistic estimate: ~3% equity risk premium going forward.

(1:02:33) Translating that into ~5% real expected equity returns.

(1:05:10) Staying optimistic: invest long-term and live modestly.

(1:05:58) The risk of pessimism: losing purchasing power in safe assets.

(1:08:06) The evolving role of bonds as diversifiers.

(1:09:55) Why market timing is a losing strategy.

(1:11:00) Elroy's definition of success: happy children and grandchildren.

Links From Today's Episode: Meet with PWL Capital: https://calendly.com/d/3vm-t2j-h3p

Rational Reminder on iTunes — https://itunes.apple.com/ca/podcast/the-rational-reminder-podcast/id1426530582. Rational Reminder on Instagram — https://www.instagram.com/rationalreminder/

Rational Reminder on YouTube — https://www.youtube.com/channel/ Benjamin Felix — https://pwlcapital.com/our-team/

Benjamin on X — https://x.com/benjaminwfelix

Benjamin on LinkedIn — https://www.linkedin.com/in/benjaminwfelix/

Benjamin Warwick on LinkedIn - https://www.linkedin.com/in/braden-warwick-a40b48a3

Editing and post-production work for this episode was provided by The Podcast Consultant (https://thepodcastconsultant.com)