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

[SR-SUPPORT]

Data Viewer reports use two distinct time bases for their variables. Understanding the difference is critical when performing manual 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 relative to the start of the current trial (where Trial Start = 0).
  
• EDF Time: Boundary variables (e.g., TRIAL_START_TIME, IP_START_TIME) are measured in absolute milliseconds since the EyeLink Host PC software was launched.
  

How to Calculate Event Times Relative to a Stimulus
A common analysis requirement is to determine the timing of an event (like a fixation) relative to a specific stimulus onset, which is often marked by an Interest Period (IP). Because the variables use different time bases, you cannot subtract them directly.

The easiest method is to export your report and perform the calculation in a spreadsheet (e.g., Excel). To find the start time of a fixation relative to the start of an Interest Period, use the following formula:

Code:

RelativeTime = 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 calculates the start of the Interest Period relative to the start of the trial. It effectively converts the IP's start time from "EDF Time" to "Trial Time."
   
2. CURRENT_FIX_START - [Result of Step 1]
   Now that both values share the "Trial Time" base, this subtraction yields the fixation's start time relative to the Interest Period.