什么是Scroll depth？
How far down a page visitors scroll, usually reported as the percentage of the page height reached, such as 25, 50, 75, or 100 percent.
Scroll depth measures how far down a page a visitor scrolls, typically reported at thresholds like 25, 50, 75, and 100 percent of the page height, or as an average depth per page. It answers a question pageviews cannot: did anyone actually read this?
It is most useful on long pages. If 80 percent of visitors never pass the halfway mark of a landing page, the content below that point may as well not exist, including any call to action placed there. Common pitfalls: short pages show 100 percent depth from a single wheel tick, percentage depth is not reading (people skim past text quickly), and lazy-loaded content changes the page height mid-scroll, which skews percentage thresholds.
Scroll depth also rescues session duration on single-page visits, since scroll events prove the visitor was active even when no second pageview fires.