This file contains information on the data reported in "Estimation and Interpretation of 1/f Noise in Human Cognition" (Wagenmakers, Farrell, & Ratcliff, 2002). This file provides the following information: (1) a description of the method used in the experiment, (2) a description of the structure of the datafile 1. Method Participants. Six students of Northwestern University participated for a small monetary reward. Materials, Design, and Procedure. The stimuli were the Arabic numerals 1, 2, 3, 4, 6, 7, 8, and 9, presented at the center of a computer screen until a response was registered. A practice phase of 24 stimuli preceded the experimental phase that contained 1024 stimuli, each number stimulus occurring equally often in an order that was randomized for each task and for each participant. The experiment featured two within-subject manipulations: type of task and response-stimulus interval (RSI). First, the three tasks used were (1) simple RT, in which participants were instructed to press the '?/' key with their right index finger as soon as they detected the stimulus. To prevent anticipatory responding (e.g., Snodgrass, Luce, & Galanter, 1967) we used a variable RSI (see below for details) and presented feedback ("TOO FAST") for two seconds following very fast responses (i.e., < 100 ms), in addition to specifically instructing participants to avoid anticipations. (2) choice RT, in which participants were instructed to press the '?/' key with their right index finger in response to an even number, and press the 'z' key with their left index finger in response to an odd number, doing this "as fast as possible without making errors". (3) one second time interval estimation, in which participants were instructed to press the '?/' key with their right index finger to mark an estimated time interval of one second after stimulus onset. As in Gilden (2001), no specific instructions were given with respect to counting. The second within-subjects manipulation concerned RSI. Each task (i.e., simple RT, choice RT, and time interval generation) was performed both with a relatively short RSI and with a longer RSI, thus yielding a total of 3 (i.e., task) x 2 (i.e., RSI) = six sessions for the entire experiment. None of the participants performed more than one session a day. One set of RSIs was randomly drawn from a uniform distribution that extended from 200 ms to 600 ms. The set of long RSIs was obtained by adding a constant 600 ms to the set of short RSIs, and hence the long RSIs varied between 800 ms and 1200 ms. The order of the RSIs was randomized for each task and for each participant. We would like to stress that the only difference in the tasks (i.e., simple RT, choice RT, and generation) is in the instruction provided to the participants, the single exception being the practice phase of the generation task, in which participants received feedback on their responses (i.e., their estimated time in milliseconds). The order of the tasks was determined using a counterbalanced design. 2. What's in the data file Let's take a look at part of the data file from subject 1 in the choice RT task (s01cs.out): / 542 2 0 689 6 / 426 2 0 385 8 / 501 2 0 796 6 / 403 2 0 598 8 z 497 2 1 638 3 The first field (i.e., "z" or "/") gives the key that was pressed in response to the stimulus. The second field gives the observed RT (i.e., the dependent variable). The third field can be ignored (it's always 2 since the 24 practice files for which the code equals 1 have already been eliminated from this file). The fourth field denotes whether the stimulus is odd or even (0 = even, 1 = odd). The fifth field gives the RSI (Response-Stimulus Interval, that is, the time between the participant's response on trial 'n' and the presentation of the subsequent stimulus corresponding to trial 'n+1'). The sixth and final field gives the stimulus used (i.e., one of the numbers 1, 2, 3, 4, 6, 7, 8, and 9).