The limited duration of short-term memory (~18 seconds without a form of memory rehearsal) quickly suggests that its contents spontaneously decay over time. The decay assumption is part of many theories of short-term memory, the most notable one being Baddeley's model of working memory. The decay assumption is usually paired with the idea of rapid covert rehearsal: In order to overcome the limitation of short-term memory, and retain information for longer, information must be periodically repeated or rehearsed—either by articulating it out loud or by mentally simulating such articulation. There is also another type of rehearsal that can also be used to improve short term memory is attention based rehearsal. The information is mentally searched in a particular sequence or list. The information is likely to re-enter the short-term store and be retained for a further period using either of these ways