The first goal of this study was to identify correlates of typing skill in younger and older adults. Research has shown that a strong predictor of typing speed is how far ahead in the text one can look. Salthouse examined this possibility by occluding text in advance of the letter being typed. Another finding is that meaningful text is typed more rapidly than non-sense text. That is, "fast typists may be fast in part because they are better able to predict and behave appropriately in response to lengthy sequences of characters" (p. 346).
Related to this is the observation that all keystrokes are not executed at the same rate. Salthouse explains that skilled typists may be more consistent in keying speed, and that there are two sources of variability related to keying speed—interkeystroke and intrakeystroke—and two sources of this variability—mechanical and experiential. Mechanical sources of variation are related to keyboard layout and alternative of keystrokes with different hands. For example, the interkey interval is 30-60 ms shorter for keystrokes with opposite hands. Experiential sources of variation are related to typing experience. For example, there is a negative relation between interkey interval and frequency of letter combinations—that is, letter combinations that occur more frequently in English words are typed more rapidly than less occurring combinations. Comprehension of material is another example of an experiential factor. Another example is the word initiation effect. The latency for the initial letter in a word should be related to the time required to retrieve that word from memory—and, in fact, there is a positive correlation between initial letter latency and word length.
The second goal was to examine age effects on typing. It is well established that the speed of simple cognitive operations slows with age. To the extent that things like perceptual processing speed are related to typing speed, older adults should be at a disadvantage. For example, if older adults’ RTs are slower than those for younger adults by about 100 ms, then older adults should type about 20 wpm slower than younger adults. However, Salthouse notes that differences of this magnitude are unlikely, for a number of reasons. First, typing is faster than RT. However, the fact remains that RT declines with age and is negatively related to typing speed (faster RTs, faster typing). Salthouse recommends the molar-equivalence molecular-decomposition approach as a strategy for investigating this paradox. Samples are obtained in which older and younger adults are matched on molar level performance (such that the age-proficiency correlation is near zero), and then age differences on molecular level processes are investigated. Two distinct outcomes are possible. The first is that expected age-related declines are found for molecular processes, despite comparable overall performance levels across age. This suggests a compensatory process. The second is that experience in the activity preserves high levels of functioning on the molecular processes. This suggests a maintenance phenomenon.
The project consisted of two studies, and subjects performed a variety of typing, reading, and RT tasks. The major results were as follows. First, there were expected age-related increases in CRT, but not in typing speed. Salthouse explains that "Typing performance, therefore, appears to be maintained across the adult life span in these individuals, whereas ‘typical’ age-related declines are exhibited in choice RT and rate of alternate hand-tapping . . . " (p. 354). The second major finding concerns the eye-hand span measure. The manipulation related to this measure involved displaying 1, 3, 5, 7, 9, 11, or 19 letters of a to-be typed text. For example, in the 1 condition, only 1 letter in advance of the letter currently being typed was displayed. A related analysis showed that, relative to younger typists, older typists have 254-360 additional ms of preparation time. Salthouse argues that "The significant effect of age on the eye-hand span suggests that one mechanism to compensate for declining perceptual-motor speed with increased age is more extensive anticipation of impending keystrokes" (p. 357).
Other Interesting Results
First, eye-hand spans were larger with meaningful text, skilled typists had larger eye-hand spans with meaningless text, and there was no relation between age and eye-hand span with random letters. (What drives the increase in eye-hand span?) Second, the ratio of one finger to two hand digrams showed that the difference between the two was greater for skilled typists. The same was true for one finger to two finger ratios. A word initiation effect was found (longer inter-key latencies for word-initial letters), and this effect was smaller for skilled typists. Reading rate and comprehension (comp read-comp type) were unrelated to skill and age. There were skill and age effects on CRT tapping, and digit symbol, but not on memory span. Across studies, the best predictors of typing speed were interkeystroke variability, one finger-two hand ratio, eye-hand span, and alternate hand tapping.
Comments and Questions
Eye-hand span refers to the minimum amount of text preview necessary for normal typing speed to be achieved. Salthouse argues that span arises as a consequence of "rapid and efficient operation of parsing, translation and execution processes" (p. 367). In fact, faster typists have larger spans than slower typists, and span decreases, as when typing nonsense text. Furthermore, "the larger span among skilled typists arises because of a need for higher rates of transmission of information to the parsing, translation, and execution processes. These processes are apparently either more efficient or overlap more extensively among the skilled typists, such that there is a greater demand for information about forthcoming characters" (p. 368). Eye-hand span is a consequence, not a cause of greater typing speed.
The most interesting results pertain to age and eye-hand span. One important finding is that older typists showed expected declines in components of typing skill—CRT, tapping speed, and digit-symbol. The implication is that "even millions of ‘trials’ of experience can lead to the preservation of basic capacities" (p. 369). But, recall that the age-skill correlation in the same was zero. Thus, "a dramatic discrepancy therefore exists between the results of traditional laboratory tasks and the performance of the real-life activity of typing" (p. 369). An implication of the positive age-eye hand span correlation is that, in older adults, processes overlap more extensively, and thus compensate for declines in basic capacities. But, Salthouse concedes that it is possible that the older typists were simply much faster typists than the younger typists when they were young.