You cannot reliably measure that on the client-side, either. But if you assume a "normal" use case anyways, you could easily track the percentage of the video s

You cannot reliably measure that on the client-side, either. But if you assume a "normal" use case anyways, you could easily track the percentage of the video stream requested by a client. That should approximate percentage watched. Of course you need to have certain control over your infrastructure for the matter. If you outsourced everything to a CDN and thus have no clue what happens to your videos, well, you probably weren't that interested in the data in the first place, weren't you?