An educational psychologist is conducting a study to evaluate the effectiveness of a new mindfulness app designed to reduce exam anxiety in secondary school pupils. After a six-week trial, the psychologist conducts structured interviews with each pupil to assess and rate their anxiety levels on a standardised scale.

However, a peer reviewer points out that validity is compromised because the psychologist conducting the interviews was fully aware of whether each pupil was in the active mindfulness app group or the control group.

Explain how this could have affected the validity of the study.