This is from a 2003 NRRPT newletter explaining the process to approve a question. Maybe it will answer your question.
"Once a question is written, it is submitted for a peer
review by three members of the panel or board. This
peer review includes format and clarity, technical content
and difficulty. Each question submitted is required to
include a specific reference citation which provides a
clear defense of the correct response. These references
are monitored closely so that questions can be
updated as regulations change.
Once the question is complete, it is put through the
Angoff process. The Angoff process addresses both the
need for quality assurance and an individual’s qualification.
The Angoff method requires expert judges (Panel
and Board members), the larger amount the better, who
individually judge each question on the basis of what
percentage of a “minimumly-qualified-person with five
years experience” will answer correctly. In the process,
both the high and low scoring judges of the question are
asked to explain the rationale for their score. Each judge
is then given the opportunity to change their score based
on the discussion or they can elect to leave their original
score. This structured review mechanism, with the
maximum number of judges involved, provides a balance
and fairness to the process.
When the 150 question exam is put together, the 150
individual Angoff numbers are averaged for a mean
rating. This result is the percentage score expected to
be achieved by a qualified candidate. The percentage
score is applied to the 150 questions to arrive at the
passing point for the exam. The passing point is
presented to the board for their approval. Once the
passing point is approved, any candidate taking that
exam and achieving that point or greater, passes the
exam."