The fundamentals of a well-built portfolio
An effective stock portfolio rests on four inseparable pillars:
- Diversification: spreading risk across several stocks and sectors so that a single disappointment cannot destroy the portfolio
- Sizing: allocating each position a size proportional to conviction and risk, not the same size to all of them
- Entry discipline: buying only stocks that meet your selection criteria, without being swept along by emotion
- Active management: rebalancing, cutting losses when the thesis breaks, letting winners run
How many stocks in a portfolio?
The question of the optimal number of stocks is one of the most debated in portfolio management. The answer depends on your capital, the time you have available and the liquidity of your investment universe.
Academic research (Statman, 1987; Evans & Archer, 1968) converges on the same result: most specific (non-systematic) risk is eliminated with 15 to 20 well-diversified stocks. Beyond that, you only eliminate marginal risk while diluting your ability to outperform the index.
Sector diversification: the key dimension
On European small caps, sector diversification matters more than you might think — spreading across industries cushions the portfolio against any single sector shock. Here is a target sector allocation for a 15-stock portfolio:
| Sector | Target weight | Number of stocks | Why |
|---|---|---|---|
| Technology / SaaS | 20–25% | 3–4 | Structural growth, high margins |
| Industrials / Manufacturing | 20–25% | 3–4 | The natural weight of Euronext Growth |
| Healthcare / Medtech | 15–20% | 2–3 | Partly decorrelated from the economic cycle |
| Services / Retail | 15–20% | 2–3 | Defensive, FCF often positive |
| Construction / Real estate | 10–15% | 1–2 | Cyclical — to underweight in periods of high rates |
| Other / Opportunities | 10–15% | 1–2 | Stocks outside the classic sectors, high conviction |
Sizing your positions
The most common mistake retail investors make is to give every position an equal size — for example 5% per stock in a 20-stock portfolio. This approach ignores the fact that not all opportunities are equal.
A dynamic allocation is built on conviction:
- Full position (6–8%): strong signal, very clear thesis, decent liquidity
- Normal position (4–5%): average signal, good composite score but a less obvious catalyst
- Watch position (2–3%): interesting but with doubt over the thesis, or insufficient liquidity
The Kelly criterion lets you compute these sizes mathematically, by combining the probability of a gain (from the ML model) and the screener's historical win/loss ratio.
Rebalancing and active management
A portfolio is never set in stone. Three events trigger a review:
- The thesis is broken: the financial results invalidate the initial assumptions (growth halted, debt exploding, loss of a key customer). You should exit quickly, even at a loss.
- The price target is reached: a stock that represents more than 15% of the portfolio after a sharp rise should be trimmed — back to 7-8% and lock in the gains.
- A better opportunity is identified: replace a low-conviction position with a stock that has a higher composite score.
The 20% rule: On European small caps, a dynamic stop-loss at −20% from the entry point is a useful discipline rule for beginners. It prevents catastrophic losses on companies whose thesis has deteriorated. It does not replace fundamental analysis but protects against errors of judgement.
The 5 classic mistakes of the beginner portfolio manager
Buying without defined criteria
Buying a stock because it was "recommended on Twitter" or because it has risen a lot is the recipe for buying at the top. First define your entry criteria (minimum score, permitted sectors, minimum liquidity) and only buy if the stock meets them.
Never cutting your losses
Holding a losing stock indefinitely "in the hope it recovers" is a documented cognitive bias (loss aversion). If the fundamental thesis has changed, the right decision is to exit and reinvest in a better opportunity.
Concentrating in a single sector
Holding 80% of your portfolio in technology or healthcare exposes you to massive sector risk. A piece of regulation, an interest-rate crisis or a sector shock can destroy half of an undiversified portfolio in a few weeks.
Ignoring small-cap liquidity
On Euronext Growth and Access, a €10,000 position can represent several days of volume on some stocks. In the event of an urgent exit, the price can fall by 5 to 10% simply because of your own selling.
Over-trading — too much turnover
Transaction costs and taxes quickly eat into the performance of a high-turnover portfolio. The best-performing small-cap investors often have holding periods of 12 to 36 months. Tax-advantaged accounts (such as the PEA-PME in France) can further reduce the drag for eligible stocks.