Why does the curriculum say some trend growth changes are easy to forecast while others are impossible? How do I tell the difference?

The curriculum draws a critical distinction between two sources of trend growth change — and understanding this distinction is essential for CME.

Category 1 — Forecastable Trend Changes (Demographics):

Demographic trends are the most forecastable driver of trend growth because the data is available decades in advance. People who will be in the workforce in 20 years have already been born.

Example — Ashford Capital's Japan Allocation:

Ashford is setting 20-year CMEs for Japanese equities. Demographic data shows:

• Working-age population (15-64) declining 0.7% per year
• Over-65 population growing from 29% to 36% of total
• Total fertility rate at 1.2 (far below replacement 2.1)

This implies trend real GDP growth of approximately 0.5-1.0% — well below the 3-4% of the 1980s. Ashford can incorporate this with high confidence because the demographic trajectory is essentially locked in. Immigration policy could alter it, but the baseline is clear.

Category 2 — Unforecastable Trend Changes (Exogenous Shocks):

By definition, truly exogenous shocks are not built into asset prices in advance. The risk of such events is reflected to some degree (markets price in general uncertainty), but the specific timing, nature, and magnitude cannot be forecast.

The Practical Asymmetry:

The Retrospective Identification Problem:

The curriculum makes an important point: even after an exogenous shock occurs, it's difficult to identify, assess, and quantify its effect on trend growth until the change is 'well-established and retrospectively revealed in the data.' This creates a dangerous lag — the trend may have shifted years ago, but your CME still reflects the old trend because the data hasn't confirmed the change yet.

Example — Post-GFC Trend Growth:

After the 2008-09 financial crisis, it took roughly 5-7 years before the data clearly showed that US trend growth had shifted from ~3% to ~2%. Analysts who continued using 3% trend growth for their equity CME throughout 2010-2015 systematically overestimated long-run returns.

Practical Approach:

1. Forecastable factors: Build directly into your central CME estimate. Demographics, educational attainment, and institutional quality change slowly enough to model with reasonable confidence.
2. Exogenous shock risk: Handle through scenario analysis. Don't try to predict WHICH shock will occur, but acknowledge THAT shocks occur and assign meaningful probability to extreme outcomes.
3. After a shock occurs: Monitor for evidence of permanent trend change vs. temporary disruption. Be willing to revise trend growth earlier than the data might 'confirm' — waiting for retrospective confirmation means your CME is always lagging.

Test your understanding in our CFA Level III question bank.