Online psychology experiments: calibration 2 — size

Cognitive psychology papers that report computer-based experiments often specify the distance of a participant to the monitor, the size of the monitor, or the visual angle of a stimulus. This may prove a problem for some online experiments, where monitor sizes may vary from 13″ to 24″ — and beyond. Subjects may be sitting very close to their display, as often happens with laptops, or very far from their display, as often happens when the machine is connected to an HDTV. Asking a participant to measure the size of their monitor (“17” inch screens can vary in size) or how far their eyes are from the screen, may limit participation. It would be useful to have the equivalent of both of these measures in order to be able to properly assess results.

Given the size of the monitor and the distance to the screen, many experiments could compensate by resizing their stimuli. If such measurements were available, the presentation package could be designed in a way that any stimulus presentation could be cleanly altered in size. Writing the stimulus presentation package in a way that allows for coding relative, rather than absolute, coordinates would obviously assist with this.

How are these measures to be achieved, if not with time and tape measure? The classic result that the thumb appears to be 2 degrees of visual arc wide when held at arms length could be useful here (Groot, Ortega, & Beltran 1994; O’Shea 1991).

We could ask the participant to hold their thumb at arm’s length, covering a single dot. Their free hand would be used to adjust the size of the dot, until their thumb exactly covered the dot. They begin by making the dot smaller than their thumb. They increase the size of the dot until they can just see it. Cursor keys work well for this. The rest of the stimuli can then be adjusted to fit. I programmed something similar using a plastic keypad in an fMRI experiment (Tovey, M., Whitney, D., & Goodale, M. 2004), and it works well.

A second possibility would be to ask the subject to hold their thumb out at arms length so that it just covers one of, say, fifteen dots on the screen, each of a slightly different size. The only thing the participant would then have to do is click on the dot closest to their thumb size.

Addendum: What problems might these approaches encounter? What other difficulties might come about due to variations in hardware and environment?


Groot C, Ortega F, Beltran FS. (1994). ‘Thumb rule of visual angle: a new confirmation‘. Perceptual & Motor Skills. 78(1):232-4.

O’Shea, RP. (1991). ‘Thumb’s rule tested: visual angle of thumb’s width is about 2 deg‘. Perception. 20(3) 415 – 418 doi:10.1068/p200415

Tovey, M., Whitney, D., & Goodale, M. (2004, February). Blind Spot Retinotopy: a control considered. Cognitive Science Research Seminar, Carleton University, Ottawa.


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