Start with your conversion rate, obviously, but that’s just the beginning. Track bounce rate, time on page, and scroll depth to see engagement. Look at click-through rates on CTAs, form completion rates, cart abandonment (for e-commerce), and lead quality (for B2B). If you’re running A/B tests, keep an eye on statistical significance, lift, and confidence levels. Segmented data is even more powerful; it tells you how returning users convert vs. new ones, or which devices or channels underperform.