It should be noted that several strategies have been used in studies of ICBT for GAD to identify people with clinical or subclinical levels of GAD. These include manually administered clinical interviews, automated structured diagnostic interviews, or cut-off scores based on responses to standardised questionnaires which are usually administered online. Where possible, these different criteria are noted below.

With respect to measurement of symptoms and severity of GAD in trials of ICBT, most studies have reported the use of the Penn State Worry Questionnaire (PSWQ) (Meyer et al. 1990) or the Generalized Anxiety Disorder 7-Item Scale (GAD-7) (Spitzer et al. 2006). The PSWQ consists of 16 items and is considered a valid clinical measure of worry characteristic of GAD. Early psychometric evaluations revealed the PSWQ had high internal consistency and temporal stability (Meyer et al. 1990) and was able to differentiate patients with GAD from those with other anxiety disorders (Brown et al. 1992). The GAD-7 is an increasingly popular measure of symptoms of GAD, which comprises seven items from the DSM-IV criteria for GAD. Psychometric evaluations of the GAD-7 indicate it is sufficiently sensitive to detect GAD, panic disorder, social phobia and post-traumatic stress disorder. A recent psychometric comparison of the PSWQ and GAD-7 revealed a moderate correlation between the two scales, and both were sensitive to change, although the GAD-7 appeared to be more sensitive and may, therefore, confer some advantages in clinical work (Dear et al. 2011b).