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As of version 1.70.00, the onset and offset times of stimuli can be defined in several ways.
Start and stop times can be entered in terms of seconds (time (s)), by frame number (frameN) or in relation to another stimulus (condition). Condition would be used to make Components start or stop depending on the status of something else, for example when a sound has finished. Duration can also be varied using a Code Component.
If you need very precise timing (particularly for very brief stimuli for instance) then it is best to control your onset/duration by specifying the number of frames the stimulus will be presented for.
Measuring duration in seconds (or milliseconds) is not very precise because it doesn’t take into account the fact that your monitor has a fixed frame rate. For example if the screen has a refresh rate of 60Hz you cannot present your stimulus for 120ms; the frame rate would limit you to 116.7ms (7 frames) or 133.3ms (8 frames). The duration of a frame (in seconds) is simply 1/refresh rate in Hz.
Condition would be used to make Components start or stop depending on the status of something else, for example when a movie has finished. Duration can also be varied using a code component.
In cases where PsychoPy cannot determine the start/endpoint of your Component (e.g. because it is a variable) you can enter an ‘Expected’ start/duration. This simply allows components with variable durations to be drawn in the Routine window. If you do not enter the approximate duration it will not be drawn, but this will not affect experimental performance.
For more details of how to achieve good temporal precision see Timing Issues and synchronisation