Discussion of the recently published IMS Caliper specification. Caliper describes the method to encode and transfer clickstream data - that is, a recording of each action taken by a user (click of a mouse, press of a keyboard). This data iss especially useful for user interface design. Note that this is not the same as the IMS Caliper RAM (Real-time Analytics Messaging) specification “to implement real-time, actionable messaging alerts”. which has changed its name to IMS HED Analytics group. The article also raises questions about the interoperability between IMS Caliper and ADL's xAPI (Experience API). "One of the first tweets with the #imscaliper hash tag after the publications read 'consider current disparity between #xapi & #imscaliper, interoperability standards that don’t interoperate'."