Metrics for USB Image

Originally created by @Anonymous on #16009 (Redmine)

Compute metrics on the downloads of the DVD and USB images
- assuming the infra to gather the raw data is done as part of sajolida’s UX core work in advance (tails#14922 (closed))
- deliverable: nicely formatted data we can report to the sponsor, once before we ship a USB image, once a few months later

I.1 Number of downloaded images and fraction of USB images

In this task, we will compute the number of images downloaded from our
website at the beginning and at the end of this project. At the end of
the project, we will also compute the number of downloads of the new USB
images.

I.2 Conversion rate of Windows and macOS users

As part of our installation instructions, people are instructed to open
the instructions on another device (for example a smartphone) so they
can continue reading them while restarting their computer on Tails for
the first time. Doing web analytics on the reopening of our installation
instructions can be used as a metric of successful installations by
first-time users.

In this task, we will compute the conversion rate of people following
our installation instructions for Windows and macOS, both before and
after the release of USB images to our users (Milestone #3).

Related issues

• Related to tails#16078 (closed)
• Related to tails#15312 (closed)
• Blocked by tails#14922 (closed)