I'm writing a script that presents two short videos on the screen in each trial. The videos are in .mp4 format. The script runs locally on my Mac and online on my Mac, but PC users tell me that they get this message when trying to run the script online:

Inquisit Media Error: Cannot play back the file: the format is not supported. line 77: file win\MediaPlayerControl.cpp
Unable to load the media file 'C:/Users/MiaSW/AppData/Local/Temp/plus2.mp4'. Verify the file is of a supported video format.

I've managed to figure out that only version 12 of Windows Media Player can play .mp4 files, and that most people don't have version 12. Any ideas what video file format Inquisit can handle that can also play on PCs?

Under Windows, Inquisit 4 relies on specific codecs available on the system to render videos (so-called DirectShow filters, to be precise). If no suitable codec is available on the given system, the video will fail to render with the error message you observed. And it is also true that older and/or not regularly updated Windows versions may not support the some "modern" video formats. Formats that should be largely fail-safe under typical, including older, default Windows installations would be WMV or MPEG-2.

(cf. the "Remarks" section in the language reference for the <video> element https://www.millisecond.com/support/docs/v4/html/language/elements/video.htm , and https://www.millisecond.com/forums/FindPost9931.aspx if you wish to administer different formats to different platforms).

Hope this helps.