When writing numbers, should we use numerals ("5,000") or words ("five thousand")? I'm particularly interested in how easy each is to read and interpret, and whether it's an important consideration for dyslexic users.
In most cases it's preferable to use numerals. Numerals have a number of advantages that aid readability:
*I had a short look for some data on how reading numbers is affected by dyslexia. Obviously there is a big chance you're going to have issues with numbers in terms of sequencing and associating meaning to symbols. See also the International Dyslexia Association on the impact on math. But it doesn't seem to always be the case. In fact, apparently we process text and Arabic numerals using mostly different and separate pathways in our brain (studies from Oxford Journals and Taylor&Francis). This would suggest that if your dyslexia affects the reading of Arabic numerals, having to deal with the simpler shorter strings of the numerals would still be preferable over numbers in words. But there is a chance that some people with dyslexia will be able to deal with Arabic numerals much easier than words.
Personally, I find numerals easier to read, especially when there are many numbers being displayed at once. If the numbers are in a table or other context where they could be compared to other numbers, numerals are the only appropriate choice. When the number appears in a block of text, however, it can be spelled out or represented with numerals.
A common stylistic practice for displaying numbers within text is to spell out smaller numbers, eg, zero through twenty, while using numerals for larger numbers that have longer or hyphenated spellings, eg, twenty-one.
Another consideration is whether the numbers are approximate or exact. I might say ten or twenty but finally settle on 17. It is probably best though not to mix representations within the same sentence.