A clinical psychologist has developed a new 10-item questionnaire to assess the severity of generalised anxiety disorder, called the GAD-10. To establish the validity of this new scale, she administers it to a sample of 150 patients alongside the Beck Anxiety Inventory (BAI), a well-established and widely accepted diagnostic tool. She calculates the correlation coefficient between the scores from the two measures and finds a strong positive correlation ($r=+0.87$).

Which type of validity is the psychologist primarily establishing?