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I'm working on a set of guidelines for the use of user interface elements. I'm currently working on ordered lists (<ol> elements) and I have a couple of questions which I've been unable to find a definitive answer to:

  • Under what conditions is it more appropriate to use numbers (decimal, lowercase roman, uppercase roman) VS letters (lowercase, uppercase) to number list items?
  • What's the difference (if any) in terms of usability between the different types of numbers and letters available?

I've searched for previous research / data on this, but I've been unable to find relevant information. Any help or friendly points in the right direction would be greatly appreciated!

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    I'm not sure if my opinion is shared by any/most people, but I tend to think of numerals as representing sequence or priority, and letters as representing things that are equal (i.e. not an "ordered" list at all, but one that still provides readers with a name/key for each item). "Step A," for example, sounds strange to me, as does "Priority A." On the flipside, "Option A" or "Option 1" both sound fine to me. – Nate Green Aug 12 '16 at 19:36
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In my humble opinion, decimals are the most natural and intuitive way to number lists. We learn the decimal symbols and their meanings from very early age. We become very familiar with them, so the mental representation is automatic, quick and effortless. The mental representation of the other numbering schemes require slow conscious effort, especially when numbers are big (i.e. 21), which increases cognitive load and error rates.

Other factors in my opinion that affect the numbering are :

  • The contents of the list items. If the list items are numbers, maybe the decimal numbering will create confusion.
  • The length of the lists. If you have very long lists, then it would be very counter-intuitive to use roman or letter numbering.
  • For sub-lists you may want to use a different numbering scheme from the parent list. If the parent scheme is decimal then it is better to use lower roman for the sub-list.
  • The general context of the site. You have to use a numbering scheme that is consistent with the whole site, and doesn't conflict with other elements of the site.
  • The goals of the site. If the most frequent use case of the list is finding the 21st item on the list or how many positions is item A higher than item B, then numbering is the only option.
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(Not based on research, but based on my own perceptions and gut feelings)

The types of ordered lists communicate different things to a user. Bullets, numbers, and letters can be used differently to show different relationships between the items

Use numbered lists when:

  • a strict order is implied (for example: steps in a how-to guide, stages in a process)
  • appealing to an international audience (numbers are more universal)

Use letters when:

  • appealing to only a fluent English-speaking audience
  • lists are small (seems less comfortable to enumerate with lists that get into "S. ..., T. ..., U. ...". Also, lists > 26 items will have "AA. ... AB. ..." and that's getting really awkward)

Use bullets when:

  • the information can be presented in any order (the data points have no priority over each other)
  • I would add that if you are considering using bullets (because the data points have no priority over each other) then you should probably be using an unordered list, not an ordered one. – Rick Lecoat Aug 14 '16 at 7:26
  • @RickLecoat Yes, bullets are a type of unordered list. I just included it for the sake of being thorough. – maxathousand Aug 14 '16 at 7:52
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I use numbered lists when order matters. For example: Step 1 must be performed before step 2. I would then use lowercase Roman ordinals (i, ii, ...) or sub-numbered ordinals (1.1, 1.2, ...) for nested numbered lists.

I prefer using lettered lists when items represent alternative choices. For example: Plan A vs. plan B.

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In my experience (especially with academic writing) I would use letters to indicate a category that I can later refer to, rather than a sorting or hierarchical order. Within lists with letters I also would use roman numbering to indicate subcategories, as in i) ii) iii) iv) and so on. While within lists with numbers is have a .# as subcategory as in 1 1.1 1.2 2 2.1 2.2 and so on. Next to numbers I also usually see dots rather than brackets, whilst letters are usually followed by brackets.

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