Dimensions, Units and Calibration
How to think about the dimensions of your stimuli.
All meadows tasks that involve stimuli allow you to specify their size in a relative (percentage) unit. This means that the stimuli will take up a given fraction of the pixels in the browser window. Where appropriate, some tasks also allow specifying the size in Centimeters or Degrees. Browsers do not provide the participant's physical screen size (in inch or cm) because of security and privacy regulations. However we can ask the participant to provide us with this information.
One way to estimate the size of the participant's screen is with the Card Measurement Task. Note that if you don't include this task, Meadows will assume a default pixel density of 40 pixels per centimeter (~102dpi).
Visual Angle in degrees
This is a popular way to specify stimulus dimensions in neuroscience related tasks, but it requires that we know how far the participant eyes are from the screen. A trick to estimate this distance is by finding the blind spot using the Virtual Chinrest Task. This in turn benefits from an accurate pixel density, so it should be preceded by a Card Measurement Task (see above). If you don't include this task, Meadows will assume a distance of 40cm (15.8") to the screen.
Note that these tasks, as any task, are subject to measurement noise and participant compliance. We therefore recommend checks, such as asking the participant to provide their best guess of the screen size (e.g. 15" laptop screen, 17" etc). These can then be compared to the estimated values to get a measure of reliability. For larger studies it may also make sense to do a pilot study where you gather a sample of estimates in the population under study, to get a sense of both the variability and range.