Journal of Memory and Language 47, 145–171 (2002) doi:10.1006/jmla.2001.2835 0749-596X/02 $35.00 © 2002 Elsevier Science (USA) All rights reserved. 145 Phonetic Biases in Voice Key Response Time Measurements Brett Kessler and Rebecca Treiman Wayne State University and John Mullennix University of Pittsburgh at Johnstown Voice response time (RT) measurements from 4 large-scale studies of oral reading of English monosyllables were analyzed for evidence that voice key measurements are biased by the leading phonemes of the response. Words with different initial phonemes did have significantly different RTs. This effect persisted after contribu- tions of nine covariables, such as frequency, length, and spelling consistency, were factored out, as well as when variance associated with error rate was factored out. A breakdown by phoneme showed that voiceless, posterior, and obstruent consonants were detected later than others. The second phonemes of the words also had an effect on RT: Words with high or front vowels were detected later. Phoneme-based biases due to voice keys were large (range about 100 ms) and pervasive enough to cause concern in interpreting voice RT measurements. Techniques are discussed for minimizing the impact of these biases. © 2002 Elsevier Science (USA) Key Words: voice key; response time measurement; naming task; oral reading; measurement bias. Much research in psychology uses measures of vocal response latencies to make inferences about underlying processes. In some experi- ments, the time that it takes to initiate pronunci- ation of a printed word is used to study the processes involved in reading and word recogni- tion (e.g., Forster & Chambers, 1973). Other studies use the time to start repeating a spoken word to shed light on auditory word recognition (e.g., Connine, Mullennix, Shernoff, & Yelen, 1990). In still other experiments, spoken re- sponses are used to examine higher levels of language comprehension (e.g., Stanovich & West, 1983). By having participants name pic- tures instead of printed words, researchers can address issues about how speakers retrieve and produce words (e.g., Griffin & Bock, 1998). Vocal response times are used in studies of memory as well (e.g., Scarborough, Cortese, & Scarborough, 1977). The naturalness of spoken responses makes such tasks well suited to a vari- ety of populations and a variety of issues. By far the predominant technology used for determining voice response times, and the one used in the studies cited above, is the voice key. A voice key is a device that determines voice onset in real time. The voice key is connected to a microphone, which converts sound pressure (the physical correlate of the amplitude of the sound) into voltage. When a stimulus has been presented, a computer arms the voice key, which begins monitoring the microphone. When the sound pressure reaches a predefined target, the voice key is triggered. It notifies the computer, which stores the number of millisec- onds that elapsed between arming and trigger- ing the voice key. The typical voice key is trig- gered as soon as the sound pressure reaches a certain level, which the experimenter can pre- define (e.g., Cedrus, 2000; Psychology Soft- ware Tools, 2000). This research was supported by National Science Founda- tion Grant BCS-9807736. We thank the anonymous respon- dents to our Internet poll; Nancy Ciaparro, Kira Rodriguez, and Joe Inman for their assistance with the research; Howard Nusbaum, David Balota, Daniel Spieler, Mark Sei- denberg, and Gloria Waters for sharing their data; and Christopher Kello, Stephen Lindsay, Ronald Peereman, and Kathleen Rastle for helpful comments on an earlier draft. Address correspondence and reprint requests to Brett Kessler, Department of Psychology, Wayne State University, 71 W. Warren Avenue, Detroit, MI 48202. Fax: (313) 577- 7636). E-mail: bkessler@brettkessler.com.