The current clickthrough rate (CTR) is the overall CTR, affected by user sessions that can go on for arbitrary lengths of time if the user (or multiple users coming back to use the same computer) maintains a logged session by visiting the Portal before their session expires (expiration time is set at 15 minutes and timer resets if the user comes back within that time).
We should probably have a second CTR metric that reflects user engagement with the Portal on first (logged) visit.