A set is a collection of objects. For example, a set of fruits might be an apple, a pear and a peach. A classic example of an item that is difficult to learn is an item that asks for the list of the members of a set. For example: What countries belong to the European Union? You should avoid such items whenever possible due to the high cost of retaining memories based on sets. If sets are absolutely necessary, you should always try to convert them into enumerations. Enumerations are ordered lists of members (for example, the alphabetical list of the members of the EU). Enumerations are also hard to remember and should be avoided. However, the great advantage of enumerations over sets is that they are ordered and they force the brain to list them always in the same order. An ordered list of countries contains more information than the set of countries that can be listed in any order. Paradoxically, despite containing more information, enumerations are easier to remember. The reason for this has been discussed earlier in the context of the minimum information principle: you should always try to make sure your brain works in the exactly same way at each repetition. In the case of sets, listing members in varying order at each repetition has a disastrous effect on memory. It is nearly impossible to memorize sets containing more than five members without the use of mnemonic techniques, enumeration, grouping, etc. Despite this claim, you will often succeed due to subconsciously mastered techniques that help you go around this problem. Those techniques, however, will fail you all too often. For that reason: Avoid sets! If you need them badly, convert them into enumerations and use techniques for dealing with enumerations