Timing precision and accuracy

Accurate timing is essential and that's why we've designed and built Psychstudio around our in-house Clockwork technology. Psychstudio with Clockwork empowers your experiments with highly accurate event scheduling and timing with millisecond precision. There are many other considerations to take into account to maximise the accuracy of scheduled and timed events within your study, some of which are detailed below.

Display refresh rate

A display on a computer monitor, laptop or smartphone is a series of static frames updated at regular intervals (called the refresh rate) to create a dynamic moving display. All screen rendering events in Psychstudio are synchronized with the frame rendering of the display. In other words, visual items (e.g. fixations, masks, images, text, visual stimuli, etc) that are to be rendered to the screen are scheduled to start on the next frame to be rendered. This ensures that the actual start time of the item and the display of that item are as precise as possible.

The display refresh rate for most smartphones and consumer/business-grade LCD monitors is 60 frames/second (1 frame every ~16.667ms). Therefore, for example, if you schedule an item to display for 10ms it will only be displayed for a single frame, but the item will be displayed for an actual time of ~16.667ms. It is worth keeping the refresh rate in mind if you need very high precision and accuracy of rendering. Setting duration times to multiples of 16.667ms can help. Display refresh rate is by far the most important consideration of those discussed here.

Display refresh cycle

The time it takes to render a frame from plotting of the first pixel at the top left, to the plotting of the last pixel at the bottom right, is called the refresh cycle. Typical monitors and laptops perform these cycles between 8ms and 16ms.

Pixel response time

The time it takes to change the color of a pixel, typically measured as the time to change from one shade of gray to another shade of gray. The measuring technique is referred to as gray-to-gray, g-t-g or gtg. Typical monitors and laptops have response times (gtg) of between 1ms and 5ms.

Display touch response times

The time taken for a touch screen display to register and act upon a touch event is referred to as the display touch response time. This is heavily product-dependent and can range from 50ms to hundreds of ms.

USB device polling rate

Devices using USB ports (typical mouse and keyboard for example) are polled for updates 125 times/second (1 update every 8ms), therefore response times of buttons (mouse activated) or keyboard key events can only be known to a precision of 8ms.

Miscellaneous

Many other factors at the participant's end can influence timing precision and accuracy, including, but not limited to, system load, web browser vendor and number of applications running concurrently.