Can Wigner and his friend give different accounts of the same measurement?

An observer inside a laboratory can hold a definite measurement record while an outside observer describes a larger quantum system without reading it. These accounts address different accessible information and operations. OPH asks which records can actually be compared, what their access maps are, and whether one joint protocol preserves the distinctions being claimed. A later interference experiment can be incompatible with treating every internal record as an untouched classical fact. Agreement therefore does not require assigning simultaneous definite outcomes to every incompatible measurement someone could have performed. The task is to construct the complete observation and intervention laws. The framework offers a relational way to state the problem; it must demonstrate the relevant experiment rather than settle it by declaring all perspectives identical.

See also Is Schrödinger's cat alive or dead?, What happens when a quantum system is measured?, How can two different descriptions of nature both be right?

Also in Chapter 19: Why Can't You Ask Everything At Once?, Chapter 20: Why Does Nature Cheat, But Only By Exactly 2√2?.