User`s guide

E-Prime User’s Guide
Chapter 3: Critical Timing
Page 72
millisecond timing or reliable refresh detection, necessitating the running of the diagnostic test
and subsequent, potential tuning of the system.
E-Prime supports basic and advanced timing needs. The experimenter must be able to identify
the true critical timing needs of an experiment and know how to analyze the data in order to verify
and report the timing precision. Specific paradigm models and sample experiments are
described and made available to meet varying precision needs including:
• Single stimulus event to response timing, the default mode for E-Prime to collect
millisecond times from the stimulus to the response.
• Critical sequence of events, such as a sequence of fixation, probe, and mask where the
inter-stimulus periods are critical, as are the stimulus periods.
• Critical and varying duration of an event in a sequence, such as manipulating the
duration of the probe to mask interval to map out a psychometric function.
• Cumulative timing of a repeating sequence of events, presenting a long series of stimuli
in a continuous performance task in which there must be no cumulative errors, where timing
can be scaled to match recording times of external recording equipment such as brain wave
monitoring.
• Continuous sequences of events at a high rate with short stimulus presentation
durations, supporting ultra fast presentation times (e.g., a new stimulus presented every
12ms) through the use of vertical refresh synchronization and image caching.
E-Prime provides software tools to log and analyze timing data. If the experimenter is using
advanced timing methods it is critical that the investigator analyze the timing data from the
experiment and report timing data in write-up of the results. The E-Prime analysis stream includes
easy coding, analysis, and plotting of timing data to debug and track down timing problems,
providing audited and interpretable timing results.
3.2 Introduction to Timing Issues
All researchers who report the results of computerized research have the obligation to
understand, control and verify timing in their experiments. Professional psychological research
reports expect an accuracy of one millisecond. However, standard programming practices on
Windows and Macintosh computers do NOT provide this level of accuracy. This means that all
researchers must be critical consumers who carefully evaluate the true timing accuracy of the
programs and packages they use in research. There are commercial packages that claim
millisecond accuracy, but actually produce findings that include errors of hundreds of
milliseconds. Using E-Prime on a well-tuned PC enables a researcher to get precise
millisecond timing and to report the true timing accurately. Timing issues are complex and
sometimes difficult to understand. Nonetheless, it is part of the obligation of all scientists to
understand and report the precision of the tools they use. As Albert Einstein put it…
Things should be made as simple as possible, but not any simpler.
In this chapter, we discuss why timing on modern computers is problematic. In section 3.2, we
provide the background needed to understand these problems. In section 3.3, we explain how to
make timing accurate within the context of E-Prime. We provide the tools to do it right, do it
quickly, and report it validly. We make a strong effort to provide methods to report data