The short term memory is coded acoustically and holds 5-9 pieces of information for up to 30 seconds. It relies on maintenance rehearsal to keep the information in there, otherwise it decays or is displaced by other things.
Elaborative rehearsal of the things in the short term memory allow them to be transferred to the long term memory, where the memories are coded semantically (by meaning) and for a potentially unlimited amount of time and a potentially unlimited amount of memories. In the long term memory the memories can be lost by retrieval failure or through interference of other memories with similar meanings.
These stages always happen in this sequence according to the MSM, and none of the steps can be skipped/bypassed.
There are strengths of this model, such as it was the first model to explain the memory system, and it influenced other psychologists to make further research and develop other memory models and theories, such as the working memory model.
The MSM is supported by evidence from other studies. For example, primacy and decency effects, from the serial position effect theory, in which when participants are presented with a long list of words they tend to remember the first few words, as they are rehearsed in the long term memory (primacy effect) and the last few words as they as still in the short term memory(recency effect). The words in the middle are lost as they either decay or are displaced by new words at the end of the list. This supports the theory that each store, sensory, short term, and long term, is separate.
The MSM is also supported by evidence from amnesiacs, for example HM has problems in his long term memory after brain surgery after suffering damage to the hippocampus. He has remembered little of his personal long term memories like his parent's deaths, however his short term memory remains intact.
Clive Wearing also suffered damage to the hippocampus and could not create new long term memories, but also had a working short term memory. However as the short term memory only lasts up to 30 seconds, anything that happened prior to 30 seconds was completely forgotten. However, in Clive Wearing’s case, his declarative memory was okay, he remembered that he had a wife, however his episodic memory didn’t work. This goes against the MSM theory and suggests that there is more than one long term memory store.
Another case study, KF, had a motorbike accident as his short term memory was limited to 1 or 2 things, and his sensory memory didn't work. However he could form new long term memories, which also goes against the linear aspect of the MSM theory, as it suggests that the sensory memory can be bypassed.
Another weakness of the MSM is that rehearsal is not essential to transfer information into the long term memory, for example we can recall how to do things such as playing a game without rehearsing the information, yet even with rehearsing we sometimes cannot recall information, such as long lists of words. This suggests that the importance of rehearsal in Atkinson and Shiffrin. The model has also been criticized by being a one way passive model.