It’s the best time to buy bullish options on the Standard & Poor’s 500 Index in 20 years, Goldman Sachs Group Inc. says.

The U.S. benchmark has a 21 percent probability of rising 5 percent in the next month, according to a model by the New York bank that looks at free-cash-flow yield, return on equity, Institute for Supply Management data and capacity utilization. The options market is pricing in only a 5 percent chance the S&P 500 will move as much.

“Call buying in this environment offers strong risk-reward and allows investors to gain upside exposure while limiting risk,” derivatives strategists including John Marshall and Katherine Fogertey wrote in a note dated March 6.

U.S. stocks just completed a third week of gains, their longest stretch this year, amid a surge in hiring and a rebound in oil. Since an almost two-year low last month, the S&P 500 has rallied 9.3 percent. Yet options investors remain less optimistic than what Goldman Sachs’s model suggests is warranted given the fundamentals, according to the note.

The bank’s GS-EQMOVE, created two years ago, estimates the probabilities of large S&P 500 moves over a one-month period. The model, which doesn’t take into account technical analysis, positioning data or short-term events, tends to be better at forecasting the potential for big swings, Goldman Sachs says.