Table 1.

Text/key attributes affecting typability, including classification and rationale

Text/Key Attribute	Description of metric	Rationale	Theme(s)
Attributes present in early typewritability indices (e.g., 1940s, 1950s)
Keystrokes per word	Total keystrokes required, divided by total words*.	Shorter words are typically typed at a faster rate than longer words (Bell, 1949).	TP
Syllables per word	Total syllables, divided by total words*.	More syllables may slow text processing and typing (Bell, 1949).	TP
Word frequency	Percentage of top 1,000 English words, following Bell (1949).	High-frequency words may be processed and typed faster due to familiarity/practice (Bell, 1949).	TP, FoU
Total keystrokes	Total keystrokes required, including shift.	Longer text may be typed more slowly due to pausing to reread (West, 1957).	TP
Total words	Number of words in the text, identified as sequences of characters separated by spaces	Text with more words may be typed more slowly due to increased processing time (West, 1957).	TP
Punctuation marks	Count of punctuation marks, e.g.,.?!	Less frequent, typically further away, and may require additional keystrokes (e.g., shift; West, 1957).	FoU, L
Uppercase letters	Count of uppercase/capital letters.	Requires shift or caps lock, which is used less frequently (West, 1957).	FoU, L
Attributes since found to affect typing behaviour (e.g., 1980s)
Hand categorisation	Number of bigrams (character pairs) that are: character repetitions; finger repetitions; hand repetitions; different hands.	Different categorisations are typically typed at different rates (Salthouse, 1984, 1986).	B, L
Bigram frequency	Average frequency of letter pairs in English.	High-frequency bigrams may be typed faster due to familiarity/practice (Salthouse, 1984, 1986).	FoU
Right-side keys	Proportion of characters’ keys on the right side of the keyboard.	Right hand is typically faster for most users (Dhakal et al., 2018).	L, B
Spaces	Count of spaces.	Spaces are generally typed faster, often by a thumb (Ostry, 1983).	L, FoU, B
Non-dictionary words	Number of words not recognised by standard dictionaries, including non-words, highly technical terms and typographical errors.	Likely typed more slowly due to unfamiliarity and text processing difficulty (Salthouse, 1984).	TP, FoU
Additional proposed attributes, not investigated previously to our knowledge
Numbers	Count of numerical digits.	Less frequent and positioned away from the vertical centre.	L, FoU
Distance from home row	Average key distance from middle row of letter keys.	Greater distance may slow typing speed.	L

Themes are Text processing (TP), Frequency of use (FoU), Layout (L), and Biomechanical (B). *The number of words is customarily calculated as the number of characters, including spaces, divided by 5, which is used here unless stated otherwise. More specific derivations of the predictors considered for the model are presented in Appendix 1