How many items can a category contain before it grows too big and should be split? Long lists are overwhelming and confusing. But it can be very easy to get lost in hierarchies with subcategories, several layers deep.
In 1956 George Miller wrote The Magical Number Seven, Plus or Minus Two: Some Limits on our Capacity for Processing Information. A great paper, but unfortunately often misunderstood and misused. The number 7 is not a limit for the number of items a category on a website should contain for the number of submenus in a menu.
There are a few different types of relationships in hierarchies. Apples and oranges both belong to the category fruit. Horses and dogs are both animals. These relationships have a clear direction: it makes sense to say “every horse is an animal”, but “every animal is a horse” is clearly nonsense. Objects can be classified at different levels of abstraction.
A different type of hierarchy is based on decomposition. A typical family consists of parents and children. A ‘set’ of parents consists of a father and a mother. Fingers are part of a hand. If a list contains items that are each other’s parts, things can quickly get confusing.
There are more types of relationships, e.g. means-ends, specifying goals and subgoals which should be met in order to achieve the goal. Nested TODO-lists frequently use this type.
Back to the category-splitting problem. A good category scheme should avoid mixing different types of relationships, at least at the same level.
Bad:
- fruit
- apples
- oranges
- animals
- meat
- fur
Secondly, a good category scheme should list items at the same level of abstraction.
Bad:
- fruit
- apples
- oranges
- horses
- dogs
And finally, the perfect list contains only items that could be grouped in one big category. WordPress lists “write, manage, design, comments”. The first three are all actions/tasks, “comments” is the odd one out.
A final note: there is some wisdom in the 7-items heuristic. Many lists that are longer are poorly constructed; most well constructed lists are rather short. If a list is longer than 7 items, there is a good chance it is confusing in other respects and should therefore be redesigned.
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