Portfolio

A portfolio is the full collection of investments owned by an individual, a fund or an institution, managed as a whole rather than one asset at a time. A portfolio can contain shares, bonds, funds, property, cash and cryptocurrencies, and its owner decides how much of each to hold according to goals, time horizon and risk tolerance.

Thinking in portfolios is the basis of modern investing. The key idea, set out by Harry Markowitz in his 1952 paper "Portfolio Selection", is that the risk of a group of assets depends on how they move together, not only on the risk of each one. Portfolio management covers choosing assets, setting their weights, spreading risk through diversification and rebalancing over time. It differs from a watchlist, which lists assets an investor follows but may not own.

Think of a portfolio as a balanced meal on a plate. You would not eat only dessert, even if it tastes best, because you need different foods for different needs. Investors spread money across different assets for the same reason: each plays a different role.

Some assets, like shares, are the main course that can grow over time. Others, like cash or government bonds, are the water on the table: dull but steady. Crypto, for many investors, is the hot sauce: a small amount can add a lot, and too much can ruin the meal.

Each holding's share of the portfolio is its weight:

• $w_i$ is the weight of asset $i$, written as a fraction or percentage.
• $V_i$ is the current market value of asset $i$.
• $\sum_{j} V_j$ is the total value of all holdings.

Diversification means holding assets that do not all rise and fall together. Shares and high-quality bonds have often moved differently, so a mix of the two has historically swung less than shares alone. Cheap ways to diversify include exchange-traded funds that track a whole stock market index. Professional managers, from hedge funds to pension schemes, report the size of what they manage as assets under management and value fund portfolios by net asset value.

Because assets grow at different speeds, weights drift away from the target. Rebalancing means selling some of what has grown and buying what has lagged, to return to the plan.

Example: an investor starts with $10,000 split 60% shares and 40% bonds, so $6,000 and $4,000. Over a year shares rise 20% to $7,200 and bonds stay at $4,000. The total is now $11,200, and the share weight is $7{,}200 \div 11{,}200 \approx 0.643$, or about 64%. To return to 60%, the target for shares is $0.60 \times 11{,}200 = 6{,}720$. The investor sells $7{,}200 - 6{,}720 = 480$ dollars of shares and buys $480 of bonds, ending with $6,720 in shares and $4,480 in bonds.

Investors rebalance on a calendar, such as once a year, or when a weight drifts beyond a set band. Each sale can trigger trading costs and taxes, and some funds, such as a rebalancing ETF, do the work automatically.

Performance is usually measured for the whole portfolio, not each holding. Common measures include total return on investment, the compound annual growth rate over several years and the return year to date. Investors also watch paper losses, the unrealised losses on holdings not yet sold, which matter for banks and insurers as well as individuals. A portfolio is one part of a person's net worth, alongside property and other assets.

Crypto investors use the same ideas with some differences:

• Volatility. Bitcoin and other tokens have had far larger price swings than shares, so even a small weight can dominate a portfolio's ups and downs. Crypto is usually treated as one of the risk-on assets.
• High correlation within crypto. Most tokens tend to fall together in a sell-off, so holding many coins is less diversified than it looks.
• Stable holdings. Traders often park funds in a stablecoin between trades, which carries its own issuer and counterparty risk.
• Custody and trackers. Holdings spread across exchanges and wallets need careful custody; portfolio-tracker apps pull balances together, and some investors automate rebalancing with trading bots.

A sound portfolio also keeps enough easily sold assets to meet needs without forced sales, the problem described under liquidity risk.

• Investor.gov (US SEC): Asset Allocation
• FINRA: Asset Allocation and Diversification