A simple two-asset, momentum-based rule: if SPY’s 60-day RSI is above 60, invest 100% in BRK/A; otherwise invest 100% in SPY. No regular rebalancing; the choice is binary based on SPY momentum.

What you’re looking at is a simple two-asset rule. A momentum gauge called RSI (Relative Strength Index) is computed using SPY’s price over the last 60 trading days. If that momentum gauge is above 60, you dedicate 100% of your investable money to Berkshire Hathaway Class A (BRK/A). If the momentum gauge is 60 or below, you dedicate 100% of your money to SPY (the S&P 500 ETF). There is no partial mixing of the two assets and no regular rebalancing once a position is chosen. The signal uses SPY as the market-wide indicator to decide whether to tilt toward a single blue-chip stock (BRK/A) or broad market exposure (SPY). For context, RSI is a momentum indicator that helps gauge whether recent price moves are stronger on the upside (values closer to 100) or downside (values closer to 0).

Rule-based two-asset strategy beat SPY out-of-sample: switch to BRK/A on strong SPY momentum, else stay in SPY. OOS return 22.48% vs 20.58%; Sharpe 1.37 vs 1.22; drawdown ~18.76%; beta ~0.91.