When you learn about similar things you often confuse them. For example, you may have problems distinguishing between the meanings of the words historic and historical. This will even be more visible if you memorize lots of numbers, e.g. optimum dosages of drugs in pharmacotherapy. If knowledge of one item makes it harder to remember another item, we have a case of memory interference. You can often remember an item for years with straight excellent grades until ... you memorize another item that makes it nearly impossible to remember either! For example, if you learn geography and you memorize that the country located between Venezuela, Suriname and Brazil is Guyana, you are likely to easily recall this fact for years with just a couple of repetitions. However, once you add similar items asking about the location of all these countries, and French Guyana, and Colombia and more, you will suddenly notice strong memory interference and you may experience unexpected forgetting. In simple terms: you will get confused about what is what.

Interference is probably the single greatest cause of forgetting in collections of an experienced user of SuperMemo. You can never be sure when it strikes, and the only hermetic procedure against it is to detect and eliminate. In other words, in many cases it may be impossible to predict interference at the moment of formulating knowledge. Interference can also occur between remotely related items like Guyana, Guyard and Guyenne, as well as Guyana, kayman and ... aspirin. It may work differently for you and for your colleague. It very hard to predict.

Still you should do your best to prevent interference before it takes its toll. This will make your learning process less stressful and mentally bearable. Here are some tips:

• make items as unambiguous as possible
• stick to the minimum information principle (many of the remaining rules in this text are based on avoiding interference!)
• eliminate interference as soon as you spot it, i.e. before it becomes your obsession (e.g. as soon as you see the word inept you think "I know the meanings of inept and inapt but I will never know which is which!")