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Controlling for variable screen sizes/resolutions when running online experiments

I’m running my first online experiment, and one participant (using a large Mac) reported that one of the images was presented partially off-screen. It is also a large image on my screen but still, it is not off-screen.

How do you account for the fact that participants will have different screen sizes etc., and the effects of this on stimulus presentation? In Builder for the stimulus presentation, “Units” is set to “from exp settings” and the size is left blank. Should I change the Units? What is the best Units to use to control stimulus size in online experiments?

Or is there something else (not related to this Units function) I need to change?

Thanks in advance!

I tend to use Height for stimuli and Norm for text. However, if you want to ask your participant how big their screen is, I’ve written Screen Scale which you can use.

Best wishes,