One explanation of forgetting is interference theory. This theory states that forgetting is caused by two sets of information – which were stored at different times – becoming confused.
There are two types of interference. Firstly, proactive interference is when old information interferes with new information and prevents it being stored properly. For example, getting a new phone number but telling people the old one by mistake.
Secondly, retroactive interference is when new information interferes with old information and prevents it being stored properly. For example, learning a new computer password then being unable to remember your old one.
Interference is more likely to happen when two sets of information are very similar.
A strength of interference theory is that it has supporting evidence from experiments. For example, McGeoch and MacDonald, who found that when participants were given a second list of adjectives after learning a first list of adjectives had poorer recall of the original list than groups who were given unrelated stimuli, or nothing, to learn.
This supports interference theory because it shows proactive interference, as the second list of adjectives interfered with the adjectives that they had previously learned.
A strength of this study is that it has high internal validity, since it was a lab experiment. This is means it was able to establish true cause and effect between the IV and the DV.
However, a weakness of interference theory is that it cannot explain all of forgetting; sometimes things can be forgotten without having similar information stored, so the interference theory does not offer a full explanation of forgetting.

Another explanation for forgetting is retrieval failure (or cue-dependency). This theory states that forgetting is caused by a lack of recall cues, which were present when the information was originally stored in memory.
Retrieval cues can be external (environmental/context) or internal (state/mood). Forgetting will happen if the context and mood state do not match at storage and recall. For example, recalling information in an exam condition when you’ve only revised in a classroom situation.
One strength of this theory of forgetting is that is has supporting evidence. For example, Godden and Baddeley found that recall of words was higher when the environment at learning and recall matched – they used scuba divers, and had divers learn and recall the words either on land or underwater.
This supports the cue-dependency explanation of forgetting because it shows that external cues (cues in the environment) increase recall.
Another strength of this explanation of forgetting is that it has useful practical applications. For example, students should study for their exams in similar conditions to those they will take the exam in.