  Conceptual tests are widely used by physics instructors to assess students'
conceptual understanding and compare teaching methods. It is common to look at
students' changes in their answers between a pre-test and a post-test to
quantify a transition in student's conceptions. This is often done by looking
at the proportion of incorrect answers in the pre-test that changes to correct
answers in the post-test -- the gain -- and the proportion of correct answers
that changes to incorrect answers -- the loss. By comparing theoretical
predictions to experimental data on the Force Concept Inventory, we shown that
Item Response Theory (IRT) is able to fairly well predict the observed gains
and losses. We then use IRT to quantify the student's changes in a test-retest
situation when no learning occurs and show that $i)$ up to 25\% of total
answers can change due to the non-deterministic nature of student's answer and
that $ii)$ gains and losses can go from 0\% to 100\%. Still using IRT, we
highlight the conditions that must satisfy a test in order to minimize gains
and losses when no learning occurs. Finally, recommandations on the
interpretation of such pre/post-test progression with respect to the initial
level of students are proposed.
