Engagement
Scroll depth
Also called scroll tracking.
Scroll depth is how far down a page a viewport traveled, usually in percent buckets.
How it is measured
The script records the deepest point reached, most often at 25, 50, 75 and 100 percent of page height. Lazy-loaded content changes the page height as people scroll.
Tarsier has no scroll tracking built in. If you want it, send one custom event per bucket from a scroll listener and count each only once per page view.
Worked example
A long interview reaches 50 percent depth in 38 percent of visits and 100 percent in 12 percent. The editors cut 800 words from the intro. Full-depth reads rise to 21 percent.
Readers reach the end only when the opening earns it.
How it differs
Scroll depth is vertical reach. Engagement time is time visible. A heatmap shows where on the page. Deep scroll with little time suggests skimming.
Common errors
Measuring against changing page height. Counting a short page as 100 percent by default. Ignoring lazy loading. Treating deep as read. Forgetting sticky footers.
In practice
Set your buckets once and compare templates that look alike.