POP is the percentage of outcomes where the position closes above breakeven at expiration. A short put at the 0.30 delta strike has roughly 70% POP. A long call at the same delta has roughly 30%. Knowing POP without knowing risk-reward is misleading: a 70% POP strategy with $5 risk to make $1 has worse expected value than a 30% POP strategy with $1 risk to make $5.
Expected value combines POP and risk-reward into a single number: (POP × max gain) − ((1 − POP) × max loss). Positive EV strategies make money over many trades; negative EV strategies bleed regardless of individual outcomes. The calculator forces you to see both — and the highest-ROI trades are usually the ones with the best EV, not the highest POP.