The lookback API can tell you any changes that happened to a particular object over a period of time. But can it tell you which user made those changes? Can you for example, determine which users closed defects in a particular month?
This information can be figured out (in a very slow and clunky) by looking through the revision information using the regular WS-API, but if the lookback API keeps track of who made a particular change, then I would think one could do this easily with the lookback API
You may use _User field that is specific to LookbackAPI. See this link.
Here is a query that includes _User in fields clause. _User cannot be hydrated.
https://rally1.rallydev.com/analytics/v2.0/service/rally/workspace/1234/artifact/snapshot/query.js?find={"_TypeHierarchy":"Defect","_PreviousValues.State":{$ne: "Closed"},"State":"Closed"}&fields=["ObjectID","_ValidFrom","_ValidTo","FormattedID","State","_PreviousValues.State","_User"]&compress=true&hydrate=['State',"_PreviousValues.State"]
Here is a quote from the docs about additional fields:
The following fields are also available if specified with the fields parameter:
_UnformattedID (If FormattedID="DE2345", then _UnformattedID=2345)
Revision information
_Revision. OID of revision record
_RevisionNumber
_User. User who made the edit
_SnapshotNumber
Name
Custom string fields
DragAndDropRank
All foreign key ObjectIDs (Workspace, Iteration, Release, Parent, Requirement, etc.)
All numeric fields (PlanEstimate, TaskActualTotal, TaskEstimateTotal, etc.)
All booleans
All date fields
Child Collections as lists of foreign key ObjectIDs: Tags, Tasks, Defects, Children, Duplicates, Predecessors, Successors Attributes for Iteration and Release (see Hydrating Iteration and Release for more details) Project name (see Hydrating Project for more details)