Progressive X Keyword
P switch alternative. When using the DMDX form (a keyword delimited by <> as opposed to the original DMTG form) N can either be an integer or a real number. If N is an integer it is a pixel value, if N is real it should satisfy 0.0 < N < 1.0 as it is multiplied by the width of the screen.
This keyword is used for word by word sentence reading tasks (using Response Contingency) where each word of the sentence is displayed individually and <ProgressiveX> making each frame advance across the screen. The first N indicates the starting position and only the first frame in an item needs the <px> keyword. Normally each progressive frame is displayed where the last frame ended, the second optional N can override this position and space things evenly across the screen.
The use of the MDSP controlled Response Contingency has been deprecated and with the addition of the <ContinueProgressiveX> keyword sentence reading tasks can now be done across multiple items.