Key Points to Critical Timing:
- No program running on a common desktop operating system can deliver accurate measurements at every millisecond.
- E-Prime provides custom solutions to obtain the most accurate stimulus presentation and response collection times from your PC. These solutions address the challenges that derive from restrictions in the following areas:
- Operating system
- Stimulus preparation needs
- Stimulus presentation limitations
- Response collection limitations
- E-Prime monitors its own performance, and flags potentially problematic trials for closer inspection and exclusion from data analysis.
What are the challenges?
There has been a long history of attempts to achieve accurate behavioral timing via computerized experimental research methods (Schneider & Shultz, 1973; Chute, 1986; Schneider, 1988; Schneider, ,1989; Segalowtz & Graves, 1990; Schneider, Zuccolotto, & Tirone, 1993; Cohen et. al., 1994; Chute & Westall, 1996). As the complexity of computer hardware and operating systems has increased, it has become more difficult to program and assess the precision of behavioral experiments. As computer operating systems have developed more capacity to perform multiple tasks (e.g., network communications and resource sharing, disk caching, concurrent processing) and more virtual tasks (e.g., virtual memory management, interrupt reflection, low-level hardware commands being routed through layers of virtual device drivers rather than direct hardware access), more effort must be expended to achieve and verify accurate timing.
Many researchers say “I want millisecond precision timing for my experiments.” It is helpful to take a moment and write down an operational definition of what that means. If you were to look at two software packages that both claimed millisecond precision, what quantifiable evidence would you want before you would be comfortable reporting scientific data with them? If you interpret millisecond precision to mean that no individual time measurement is ever off by more than a millisecond, then you cannot use modern personal computers running common desktop operating systems to conduct the research.
NOTE: The most serious problem for computerized psychological research is that the computer requires substantial time to perform actions and may halt the experiment without notice, thereby grossly distorting timing.
You have probably experienced times when your word processor occasionally pauses for a short time and then resumes. This same process can happen during an experiment.
Let’s take a simple example: assume you intend to display seventy-two bitmap images, each for 200ms (e.g., a rotating checkerboard). If you program this example in a standard programming language such as C or Java and check the timing using a stopwatch, you will likely find that the experiment takes longer than the expected 14400ms to run. When we implemented this experiment in E-Prime without taking full and proper advantage of E-Prime timing features, we also obtained undesirable results. As we’ll explore in detail below, timing results from this experiment also show the following characteristics:
- The median display time is greater than the specified display time of 200ms
- Periodically, there are large spikes in the display time for an individual image
- These large spikes occur intermittently over multiple experimental runs
Operating system and stimulus presentation hardware contribute to these undesirable timing results. E-Prime equips researchers to both effectively work with and around the functional limitations to obtain the most accurate timing possible and also to confirm the critical timing aspects of their experiment.
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