FAQ: How do events get assigned to Interest Areas if there are multiple Interest Areas?
Sometimes Interest Areas overlap, or one Interest Area may be completely contained within another. It is important to understand how fixations are assigned to Interest Areas in these circumstances.

Each Interest Area (IA) in an Interest Area Set is assigned a Label, an ID number and an Index. In the example below there are 5 Interest Areas. Their labels are Left_Face, Right_Face, Left_Eye, Right_Eye and Mouth. Left_Eye is contained within Left_Face and Right_Eye is contained within Right_Face. The Mouth IA overlaps both the Left_Face and Right_Face IAs. Their IDs are 1,2,3,4,5. Data Viewer allows different Interest Areas to share the same IDs, and also for the lowest ID to be greater than 1 - thus IAs also have an "Index" as well as an "ID". The Index is a number from 1 to the total number of IAs (e.g. 1-5 in the example below), and follows the order in which the IAs are listed in the middle panel of the Inspector Window. In the vast majority of cases (as in the example below), an Interest Area's ID and its INDEX will be identical.


Fixation events are assigned to a particular Interest Area based on the average X, Y location of all the samples which constitute that particular fixation. If the average X, Y location falls within the bounds of any Interest Area then it will be counted towards that Interest Area. When visualizing fixations in the Spatial Overlay view the Average X, Y location will be represented by the center of the circle which constitutes that fixation. If the center is within the bounds of the Interest Area then it will be counted toward that Interest Area. However, if only the edges of the circle overlap and not the center then that particular fixation it will not be counted towards that Interest Area.

If the fixation falls within multiple overlapping Interest Areas then by default it will be assigned to each Interest Area it falls within. In other words, the dwell time measures for each interest area that the fixation falls within will include the fixation's duration. This default behaviour can be changed by enabling the Include Fixation in Single IA preference (Edit -> Preferences -> Output & Analysis). If this option is enabled then (for static Interest Areas) the fixation will be assigned to the Interest Area with the lowest ID.

Note that when generating fixation or saccade reports some of the Interest Area variables will only reflect the Interest Area with the lowest Interest Area ID value in the situation where that a fixation falls within the bounds of multiple Interest Areas. The variables *_INTEREST_AREAS will return a list of all Interest Areas which in which that Interest Area fell, so if in the case of an overlap this will list all Interest Areas which that particular fixation fell within. To extract variables corresponding to each particular Interest Area regardless of overlap please generate an Interest Area Report.