Why Diversification Works
Diversification is the only true "free lunch" in investing — the Nobel laureate Harry Markowitz proved this mathematically in his 1952 paper Portfolio Selection, published in the Journal of Finance. By holding assets that are not perfectly correlated, you reduce portfolio volatility without reducing expected return.
The key insight is the difference between systematic risk (market risk — affects all stocks) and unsystematic risk (company-specific risk — can be diversified away). Research shows that owning 15–20 well-chosen, uncorrelated stocks eliminates approximately 90% of unsystematic risk. Beyond 30–40 stocks, additional diversification benefits are minimal. The CFA Institute's portfolio risk curriculum covers this with rigorous mathematical treatment.
How Many Stocks Should You Own?
This depends on your investment style, time available, and research depth:
| Portfolio Type | Stock Count | Approach | Role Models |
|---|---|---|---|
| Concentrated | 10–15 | Deep fundamental research on each; high conviction required | Buffett, Munger, Ackman |
| Focused | 20–30 | Balance between diversification and research depth; most individual investors | Lynch, Schloss |
| Diversified | 40–60 | Quantitative or systematic approach; some positions are "index-like" | Greenblatt Magic Formula |
| Index Augmented | Core ETF + 10–15 high-conviction picks | Index covers baseline; stock picks add potential alpha | Common institutional approach |
Warren Buffett, who manages a concentrated portfolio, famously noted that diversification is "protection against ignorance." For investors who cannot dedicate significant research time, broader diversification through low-cost ETFs — tracked via the ETF.com database or the Tadawul ETF listings — is entirely rational.
Position Sizing Principles
Even with perfect stock selection, poor position sizing destroys returns. Three frameworks that professional investors use:
1. Equal Weighting
Simplest approach: allocate an equal percentage of the portfolio to each position. Research from Research Affiliates shows equal-weight portfolios have historically outperformed cap-weighted indices, partly because they systematically underweight overvalued large-caps and overweight undervalued smaller companies.
2. Conviction-Weighted
Size positions based on research depth and confidence level. Highest conviction ideas get 5–10% allocation; exploratory positions get 1–2%. This is the standard approach at most active fund managers. The risk: overconfidence bias can cause investors to over-concentrate in their "best idea" at exactly the wrong time.
3. Risk-Adjusted (Volatility-Weighted)
Size positions so each contributes roughly equal risk to the portfolio, not equal capital. A high-volatility growth stock gets a smaller position than a low-volatility utility, even if conviction is equal. This is the basis of risk parity portfolios, popularized by Bridgewater Associates. The Portfolio Visualizer tool calculates position-level risk contributions automatically.
As a general rule, no single stock should exceed 10–15% of your total portfolio unless you have extraordinary conviction and have stress-tested the position against multiple adverse scenarios. Even the best investors have been seriously wrong on their highest-conviction positions.
Asset Allocation Across Markets
For investors using TruePrice.Cash across TASI, S&P 500, JPX, and LSE, the question of how to allocate across markets is as important as stock selection within them.
| Consideration | Conservative Approach | Balanced Approach |
|---|---|---|
| Home Market (TASI) Bias | 60–70% TASI | 40–50% TASI |
| International Developed (S&P 500 / LSE / JPX) | 20–30% | 40–50% |
| Currency Hedging | Minimal (SAR-USD peg reduces need) | Consider JPY and GBP hedging |
| Cash Buffer | 10–15% for opportunities | 5–10% |
The academic framework for international asset allocation is Modern Portfolio Theory (MPT). The most widely cited real-world application is the Norwegian Government Pension Fund (NBIM), the world's largest sovereign wealth fund, whose portfolio allocation methodology is publicly documented and serves as a benchmark for institutional investors globally.
Rebalancing: When and How
Over time, winning positions grow to represent too large a share of the portfolio (increasing concentration risk) while losing positions shrink. Rebalancing is the disciplined process of trimming winners and adding to laggards to restore target weights.
Calendar-based rebalancing (annually or semi-annually) is simple and effective for most investors. Threshold-based rebalancing (when any position drifts more than 5% from target weight) is more responsive but requires more monitoring.
Research from Vanguard's rebalancing study shows that annual rebalancing captures most of the benefit of more frequent approaches while minimizing transaction costs and tax friction. For Saudi investors, consult the CMA regulations on capital gains tax treatment, which differs from Western markets.
Behavioral Traps That Destroy Portfolios
Nobel Prize-winning research by Daniel Kahneman and Amos Tversky (Prospect Theory, 1979) proves that investors are not rational — they feel losses roughly twice as acutely as equivalent gains. This creates systematic, predictable errors:
The best defense against behavioral errors is a written investment process — a document that specifies your buy criteria, sell criteria, and position sizing rules before you are under the emotional pressure of a real trade. CFA Institute's behavioral finance resources provide excellent practical frameworks.
Measuring Portfolio Performance Correctly
Most investors measure performance incorrectly — comparing their portfolio to the wrong benchmark, ignoring risk, or cherry-picking favorable time periods. Here is how professionals measure it:
| Metric | What It Measures | Why It Matters |
|---|---|---|
| Absolute Return (CAGR) | Compound annual growth rate of portfolio | The baseline — but meaningless without a benchmark |
| Alpha vs Benchmark | Return above your relevant benchmark (e.g., TASI index) | Did stock picking add value over simply buying the index? |
| Sharpe Ratio | Return per unit of risk (volatility) | Rewards consistent returns; penalizes volatility even if returns are positive |
| Maximum Drawdown | Largest peak-to-trough loss in the portfolio | Reveals tail risk tolerance and worst-case historical outcome |
| Sortino Ratio | Return per unit of downside risk only | More investor-friendly than Sharpe — doesn't penalize upside volatility |
Calculate all of these for free using Portfolio Visualizer or Morningstar's portfolio tracker. For TASI-specific benchmarking, compare against the Tadawul All Share Index (TASI) performance data.
Don't ask "did I make money?" Ask "did I make more than I would have by simply buying a low-cost index fund — adjusted for the risk I took?" If the honest answer is no over a 5-year period, seriously consider whether active stock selection is worth your time, or whether a core-satellite approach (index fund core + a small active sleeve) would serve you better.
Sources & Further Reading
- Markowitz (1952) — Portfolio Selection (Journal of Finance)
- CFA Institute — Portfolio Risk and Return
- Vanguard — Best Practices for Portfolio Rebalancing
- Portfolio Visualizer — Free Portfolio Analysis Tool
- DALBAR — Quantitative Analysis of Investor Behavior
- CFA Institute — Behavioral Finance Resources
- Research Affiliates — Equal Weight Portfolio Strategy
- Tadawul — TASI Index Performance Data
- Morningstar — Portfolio Tracker & Performance Analytics
Full Series Complete 🎓
You've now completed all 15 tutorials in the Fundamental Analysis Series. You have the tools, frameworks, and knowledge to invest with discipline and confidence — across TASI, S&P 500, JPX, and LSE.
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