This questionnaire consists of a total of 93 items. The YPQ is a combination of multiple scales to measure common psychopathological symptoms in adolescence:

• Moods and Feelings Questionnaire (MFQ)
• Revised Children’s Manifest Anxiety Scales (RCMAS)
• Leyton Obsessional Inventory (LOI)
• The Behaviours Checklist (BC)
• Rosenberg Self-Esteem Scale (SES)

Each item has a response of either Always, Mostly, Sometimes or Never. The coding is as follows in the main database (with the exception of reversed items): 0 – Never / 1 – Sometimes / 2 – Mostly / 3 – Always

However, for many measures the endorsement of Always is too rare to be meaningful and has weak psychometric properties. Thus, for many of the YPQ measurements we usually combine the Mostly and Always categories, leaving a three-point scale –0, 1 and 2. For many of these measures, the sum score will be based on the three category items, therefore might not match if you were to sum the four category raw data. However, in all instances of mismatch the summed scores are more psychometrically valid and should be used. Additionally, for scales that have more than 15 questions we use the strategy that if 85% of the scale is completed there is sufficient amount of data to derive a sum score. Therefore, if one or two items were missing from the longer measures (MFQ and RCMAS), we will compute a sum score from the remaining items.

1a) Moods and Feelings Questionnaire (MFQ)

The MFQ is a 33-item self-report questionnaire that measures depressive symptoms. This corresponds to items 1-33 on the YPQ.

It was developed by Adrian Angold and the original reference is:

• Costello, E.J., & Angold, A. (1988). Scales to assess child and adolescent depression: checklists, screens, and nets. Journal of the American Academy of Child and Adolescent Psychiatry, 27, 726-737.

1b) Revised Children’s Manifest Anxiety Scales (RCMAS)

The RCMAS is a 28-item self-report questionnaire that measures anxiety symptoms. This corresponds to items 34-61 on the YPQ.

It was developed by Cecil Reynolds and Bent Richmond and the original reference is:

• Reynolds, C.R., & Richmond, B.O. (1978). What I Think and Feel – Revised Measure of Children’s Manifest Anxiety. Journal of Abnormal Child Psychology, 6(2), 271-280.

1c) Leyton Obsessional Inventory (LOI)

The LOI is an 11-item self-report questionnaire that measures anxiety symptoms. This corresponds to items 62-72 on the YPQ.

It was developed by Diane Bamber and Ian Goodyer and the original reference is:

• Bamber, D., Tamplin, A., Park, R.I., Kyte, Z.A., & Goodyer, I.M. (2002). Development of a short Leyton Obsessional Inventory For Children and Adolescents. Journal of the American Academy of Child and Adolescent Psychiatry, 41(10), 1246-1252.

Bamber et al. (2002) found there were three factors within this short scale.

• The first factor consisted of four items and measured compulsions (items 62, 65, 68, and 71) and had an internal consistency of .78 (good internal consistency).
• The second factor consisted of four items and measured obsessions (items 63, 66, 69, and 72) and had an internal consistency of .73 (good internal consistency).
• The third factor consisted of three items and measured cleanliness/contamination (items 64, 67, and 70) and had an internal consistency of .75 (good internal consistency).

1d) The Behaviours Checklist

The Behaviours Checklist is an 11-item self-report screen for symptoms of antisocial behaviour. This corresponds to items 73-83 on the YPQ, and was originally devised for the ROOTS cohort (Goodyer et al, 2010). While this measure has been used in NSPN publications (e.g. St Clair et al, 2017), no formal psychometric study of this measure has been undertaken in ROOTS or NSPN datasets.

1e) Rosenberg Self-Esteem Scale (SES)

The SES is a 10-item self-report questionnaire that measures global self-esteem or feelings of self-worth and self-acceptance. This corresponds to items 84-93 on the YPQ.

This was originally developed by Morris Rosenberg and the original reference is:

• Rosenberg, M. (1965). Society and the adolescent self-image. Princeton, N.J.,: Princeton University Press.

The original scoring of this measure was from 'strongly agree” to 'strongly disagree”. However, we have altered the scoring for several studies to agree with the remainder of the YPQ. So 'strongly agree” refers to 'always”, 'agree” to 'mostly”, 'disagree” to 'sometimes” and 'strongly disagree” to 'never”. There are also five items (items 84, 88, 89, 91, 92) which are reversed scored. These were reversed prior to evaluating this scale.