It may be a step along the path of improvement, but it's not plannable in the sense that you can understand the extent or time involved.
Allowing unintegrated work to leave a sprint is a risk, plain and simple. Avoid the assumptions that come with that decision at all costs. From: ScrumForum@scrum.org Sent: 3/19/2013 5:36 PM To: firstname.lastname@example.org Subject: RE: Hardening Sprints: good or bad? [00000307:00001125]
the danger of Hardening Sprints are that you accecpt to build technical dept during the "normal" Sprints. Because the complete Scrum Team might say we have enaugh time in the Hardening Sprint to fix that.
Sometimes you need Hardening Sprints (An Example might be: the Test Server are only available to a certain timeframe (you see this in SAP development very often)) but I don't like them anyway and I recommend not doing them as long as it is possible.