FAQ: How can I calculate time relative to the onset of a specific Interest Period?

[SR-SUPPORT]

Data Viewer reports use two different time bases for its variables, which is important to understand when performing calculations.

Understanding "Trial Time" vs. "EDF Time"

• Trial Time: Variables for gaze events (e.g., CURRENT_FIX_START, CURRENT_SAC_START) are measured in milliseconds since the start of the current trial.
  
• EDF Time: Boundary variables (e.g., TRIAL_START_TIME, IP_START_TIME) are measured in milliseconds since the EyeLink Host PC software was launched.
  

How to Calculate Event Times Relative to a Stimulus
A common analysis need is to find the timing of an event (like a fixation) relative to a specific stimulus onset, which is often marked by an Interest Period. Since the variables use different time bases, you can't just subtract them directly.

The easiest way to do this is in a spreadsheet program (like Excel) after exporting your report. To find the start time of a fixation relative to the start of an Interest Period, create a new column and use this formula:

Code:

= CURRENT_FIX_START - (IP_START_TIME - TRIAL_START_TIME)

Breaking Down the Formula
This formula works in two steps to create a common time base:

1. (IP_START_TIME - TRIAL_START_TIME)
   This part first calculates the start time of the Interest Period relative to the start of the trial. It effectively converts the IP's start time from the EDF time base to the Trial Time base.
   
2. CURRENT_FIX_START - [Result of Step 1]
   Now that both values are in the same "Trial Time" base, this final subtraction gives you the fixation's start time relative to the beginning of the Interest Period.