Forced Synchronisation

Some researchers have required participants to keep synchronised with the presented text7 _{as they type; typically an audible error beep provides instantaneous}

feedback to the participant as they perform the text entry task. When a character is entered that does not match the expected next character, the beep is sounded,

7 _{Recall that in the text entry experiment paradigm, the investigator presents subjects with text to be}

instructing the participant to resynchronise with the expected text before they continue. The participant may be required to enter the correct character before continuing (see Matias, MacKenzie & Buxton 1996; Venolia & Neiberg 1994; Isokoski & Kaki 2002; Ingmarsson, Dinka & Zhai 2004.) Alternatively, the participant may be allowed to continue without entering the correct character (see MacKenzie & Zhang 1999). In either case, however, it is not possible for the participant to correct their errors.

This methodology greatly simplifies error tabulation. Because of the forced synchronicity, error tabulation is reduced to a character-wise comparison of the presented and transcribed texts, which can be performed in software. However, there are three problems with this methodology:

1. Interrupting participants so they can realign themselves after every error has poor external validity and a negative impact on performance. There is a certain momentum or rhythm that one experiences when entering text. This is an important element of rapid text entry that is disrupted by being forced to stop and realign oneself with the expected text. Logan (1983) reports that skilled touch typists overlap the finger movements of consecutive keystrokes, and concludes that the planning and execution of keystrokes occurs (at least partially) in parallel.

2. Forced synchronicity also increases the error rate. Because a strict character-by-character comparison is used to detect errors, correct characters that follow an error are likely to be counted as errors too. This leads participants to make error “chunks”, beginning with a legitimate error and continuing for several keystrokes until resynchronisation is accomplished. Error chunks were reported by Matias and colleagues (Matias, MacKenzie & Buxton 1996).

3. Forced synchronisation prevents the researcher from obtaining information about correction strategies.

The first two negative effects listed above reduce the realism of the text entry task. Text entry and error rates observed during forced synchronisation do not generalise to normal unconstrained text entry. However, it is precisely this generalisation of experimental observations that is desired. The third negative effect suggests an even more egregious oversight. The text entry process is really the editing process, involving much more than the perfect linear input of alphanumeric symbols. By forbidding participants to correct their mistakes, researchers are missing an extremely important part of the text entry process. Card, Moran & Newell (1980) report that up to one fourth of an expert’s time can be spent correcting errors. A study by Soukoreff & MacKenzie (2003a) (reported in Chapter 8) found that 31% of keystrokes generated by typical computer users were editing functions such as backspace and cursor movements.

Text entry forms were a component of the interface of some (particularly early) tablet computers, where the primary technology for text entry was the stylus instead of the keyboard. These are essentially an electronic version of the common “fill-in the blank” form. Fields on a form (paper or electronic) indicate the specific locations that each character must be written, and so the input text is not simply a stream of characters, but also includes position information. In typical use (i.e., not part of an experiment) one is not obliged to fill-out the fields of a form (or even the characters in a field) in a specific order, and the correction of errors is normally possible (an error can be scribbled-out, and the correct character drawn in).

Several studies of stylus-based text input have used forms, where input fields appeared beneath each character of presented text (see Figure 2). (McQueen et al. 1994, McQueen et al. 1995, Chang 1994, MacKenzie & Chang 1999, MacKenzie et al. 1994a, MacKenzie et al. 1994b)

Figure 2 - Example of synchronisation in a form-based text input experiment

This figure is taken from MacKenzie & Chang 1999, Figure 1.

These text entry forms were a normal part of the interface, and the means by which regular text entry was supported on some stylus-based computers. While useful for studying stylus-based text entry, text entry forms do represent a (less rigid) form of synchronisation. The physical layout of the presented text and the input field make it relatively easy for participants to keep themselves synchronised. Error rate calculation is simplified because a character-by-character comparison can be used. As before, data regarding error correction strategies would only be observable if the experiment software supported error correction – and we have not seen any examples of this in the literature.

A variation of the text entry form technique was used by MacKenzie & Zhang (1997) in their investigation of the immediate usability of the Graffiti stylus-based text entry technology. The purpose was to judge participants’ familiarity with the Graffiti alphabet, and so participants were asked to enter the alphabet five times consecutively (one character at a time, no error correction). Because the participants were familiar with the alphabet and required to ignore errors, they naturally remained synchronised to the expected text. Errors were identified via a character-by-character comparison of the transcribed text. However, this is a highly specialised case, and this technique does not generalise to typical text entry.