The preferable keypad layout for ease of pressing small cell phone keys with the thumb.

The present study investigated the effect of keypad layout on the ease of operating small cell phones with the thumb in one-handed operations by young and elderly male and female participants. Eighteen young participants (9 males and 9 females) and 12 elderly participants (6 males and 6 females) operated 9 different keypads modeled after commercially available cordless handsets. Keypads designed by using the L9 orthogonal array differed in vertical pitch (V-Pitch: 7, 8, 9 mm) between keys, horizontal pitch (H-Pitch: 10, 11, 12 mm) between keys, the margin below the bottom row of keys (B-Margin: 5, 13, 21 mm), and phone body width (P-Width: 38, 41, 44 mm). Results concerning subjective overall usability showed the lowest scores for a V-Pitch of 7 mm and a B-Margin of 5 mm in most groups. However, for the female participants, with shorter thumbs, the increase in V-pitch did not improve operability. In the elderly participants, miskeying frequently occurred at dial keys of specific numbers. These findings suggest that the preferable keypad layout differs between different age groups and between male and female participants.