Readability of Text Scrolled on Visual Display Terminals as a Function of Window Size

Readability of text scrolled on visual display terminals was studied as a function of three different line lengths, two different character densities, and five different window heights (either 1, 2, 3, 4, or 20 lines). All three variables significantly affected reading rate, but to markedly different extents. Lines of full and two-thirds screen width were read, on average, 25% faster than lines of one-third screen width. Text appearing at a density of 80 characters per line was read 30% faster than text in a format of 40 characters per line. Text appearing in windows four lines high was read as efficiently as text in 20-line window, and text in one- or two-line windows was read only 9% more slowly than text in 20-line window. Comprehension of the passages did not vary as a function of window size, indicating that subjects maintained a constant level of comprehension by varying their reading rate. Implications of the results for mixing text and graphics and for limited-capacity electronic displays are discussed.