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Just like how there's more than one way to skin a cat, there's more
than one way to construct a portfolio. The first systematic method of
construction was Modern Portfolio Theory, put forth by Harry
Markowitz. MPT's approach is simple: choose weights for each asset that
maximize the amount of return received for the amount of risk or
volatility taken. However, trying to maximize the risk-adjusted return
of a portfolio leads to very unstable allocations, as ex-ante estimates
of return are notoriously difficult and previous return isn't a good
predictor of future performance.
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Risk parity is a popular method of investing that aims to outperform the
traditional 60/40 equity/bond portfolio by equalizing the risk of the
bond portion with that of the equity part of the portfolio and then
leveraging up the portfolio to hit the desired risk target. Using the
covariance, and volatility of the components, we can generate a
portfolio and leverage ratio for a given level of desired risk. This
works for two reasons: bonds have a low or inverse correlation to
equities and they generally have superior risk-adjusted returns as well,
and combining low correlation assets allows us to lower the volatility
of the portfolio. Bridgewater and Ray Dalio pioneered this approach in
the 90s with the introduction of his now famous All Weather fund.
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The beta exposure of a
stock is one of the first and most important statistics any investor
looks at. Though perhaps simplistic, it distills a myriad of various
properties of a stock into a single, easily digestible number. Using
beta, it becomes easy to roughly forecast how an investor's portfolio
would perform under different market conditions. Maybe out portfolio has
a beta of 1.3 and we expect the market to return 11% next year: great,
our portfolio should return around 14.3% (1.3*11%). While there's a
lot of problems with this simplistic model, in this post, we're going
to focus on just one: the beta of a stock changes significantly over
time. Consider this graph of Apple's rolling, 252-day beta:
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A retail investor would be forgiven for assuming that there are really
only two assets classes one should consider for personal investments:
equities (stocks) and fixed income (bonds). Wherever you look, these two
asset classes dominate the financial landscape. All robo-advisors and
most personal advisors implement a mix of bonds and equities for the
portfolios of their clients. The idea behind this mix is simple: bonds
are low risk and will earn you a marginal return, while equities are
riskier and will be the real engine of returns in good times. In bad
times, bonds will rise, mitigating some of the losses of your equity
portfolio: it's been long known and accepted that stocks and bonds have
a negative correlation. Tying this strategy together is periodic
rebalancing, taking money off the table during bull markets (and moving
them to the safer bonds), and deploying money to the equity portion
during bear markets. This in effect overlays a mean-reversion strategy
onto the portfolio, boosting the returns of an otherwise static
portfolio. The archetypal allocation is 60% equities, and 40% bonds,
though each investor's allocation is going to differ based on his risk
tolerance, age, and personal goals.
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