Full keyboards have smaller keys and more misses, but more determined candidates
Aliases: full keyboard · QWERTY touch · letter-determined input
What it is
A full keyboard gives each letter its own key, so the press sequence is almost determined at the letter layer: h-e-l-l-o does not have to be guessed from “which words could this key string be.” The cost is smaller keys; a slightly off landing becomes a neighbour. The dominant error is spatial neighbour substitution, not a failed set-disambiguation.
Why it happens
Twenty-six letters plus digits and symbols carve the same thumb heat map into fragments. Key width approaches the finger pad, so a hit is which side of a boundary the contact centroid fell on. Neighbour substitutions (i/o, n/m, s/d) form stable confusion pairs; the correction model treats spatial noise, not a combinatorial explosion of several letters per key. Because the letter sequence has already narrowed the word search, a language model only has to patch omissions and neighbours inside a small set; the candidate bar can be short or appear only when uncertain. A nine-key pad aims well first and then picks a word from a large set; a full keyboard risks aiming badly first and then repairs inside a small set. The two “candidates” are not the same object: one resolves reuse, the other patches aim.
Studying it
Log the offset distribution of contacts relative to key centres, and the neighbour-substitution rate, successful system corrections, and user reverts that follow. Independent variables include key width, thumb versus index, one versus two hands, and whether spatial correction is on; dependent measures include substitution errors, deletions, and candidate-bar use. Do not compare to a nine-key pad on word accuracy alone—a full keyboard may get the word right with many deletions, or get it right only because correction rewrote neighbours.
Where it stops holding
A hardware full keyboard has ridges and travel; its spatial error shape differs, and touch full-keyboard neighbour data should not be copied onto it. On a very wide screen (tablet landscape) keys grow, the spatial penalty drops, and the comparison with a nine-key pad is rewritten. For content that must be letter-exact (passwords, code, abbreviations), a determined sequence is worth more than large keys, even with more misses.
Applying it
- Use a full keyboard on fields that need exact letter sequences or whose lexicon coverage is poor, and treat neighbour substitution as the expected error to correct in place—not as a long list of homophones.
- When key width changes with orientation and grip, measure substitution on specific neighbour pairs (i/o, n/m); do not watch WPM only.
- Keep the candidate bar on a full keyboard converged by default; frequent expansion means spatial correction or key width is failing, not that a longer word list is needed.