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FAQ: How can I accurately keep track of reaction times (RTs) for participant responses?
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#1
09-02-2020, 08:39 AM (This post was last modified: 10-09-2025, 08:17 AM by Sam.)
Calculating a reaction time (RT) in Experiment Builder is a simple process of subtracting the stimulus onset time from the response time. The basic formula is:
Reaction Time = Response Time - Onset Time

As shown in the image, a clear way to implement this is to use an UPDATE_ATTRIBUTE node to store each time in a separate variable before calculating the final RT.
   

  1. A variable named KEY_RESPONSE_TIME is used to store the response time from Keyboard_Input.triggeredData.time (Response Time).
  2. Another variable, Display_On_Time, is used to store the onset time from DISPLAY_SCREEN.time (Onset Time).
  3. A variable named KEY_RESPONSE_TIME is used to store the response time from Keyboard_Input.triggeredData.time (Reaction Time).
The Stroop project (File menu > Examples) provides a working example of this process.


⚠️ Important Warning: Do Not Use .startTime
The .startTime property records when a node was entered, whereas the .time propert records when the node was actually executed. Due to pre-loading and screen refresh cycles .startTime is not the true stimulus onset time, and it should NOT be used for RT calculations.