Table 3.

Content and psychometrics of the measurement instruments (questionnaire)

	Measured characteristics or constructs	Measurement instrument	Answer categories or scales	α 1
Socio-demographics	Age
	Gender		Male/Female
	Educational level		Low/High
Disease-related characteristics	Age at diagnosis		0 years/1–5 years/6–12 years/13–16 years
	Treatment type		Pre-dialysis/Haemodialysis/Peritoneal dialysis/Kidney transplantation/
			Other
	Limitations in mobility	Medical outcomes Study (MOS) 6-Items Short Form Health Survey [42]	3-point scale: 1 = severely limited/2 = somewhat limited/3 = not limited at all	.78
Self-management and social participation	General self-efficacy	10-item General Self-Efficacy Scale [43]	4-point Likert scale: 1 = not right/2 = hardly right/3 = somewhat right/4 = totally right	.71
	Disease-related self-efficacy	16-item2 On Your Own Feet Self-Efficacy Scale (OYOF-SES) [44]	4-point Likert scale:1 = yes certainly/2 = yes probably/3 = no probably not/4 = no, definitely not	.90
	Health-related quality of life	37-item European DISABKIDS condition generic questionnaire (DCGM-37) [45] with six domains: independence (I), social inclusion (SI), social exclusion (SE), emotion (E), physical (P), medication (M); and a general score (range: 0–100)	5-point Likert scale: 1 = often/2 = quite often/3 = sometimes/4 = almost never/5 = never	I: .86
				SI: .70
				SE: .85
				E: .81
				P: .46
				M: .79
	Social participation	Rotterdam Transition Profile (RTP) [46] with seven life areas: school/work, finances, (independent) living, (intimate) relationships, leisure, and mobility	Four transition (to adulthood) phases (0–3)3	na4
Camp COOL	Influence of living with the condition	10 items Effects of CC Scale See Additional file 1.	5-point Likert scale: 1 = completely disagree/2 = disagree/3 = somewhat agree/4 = agree/5 = completely agree	.92
	Value of peer-to-peer (i.e. buddy-to-attendee) support	Value of peer-to-peer support (2 items for buddies and 2 items for attendees) See Additional file 1.	5-point Likert scale: 1 = completely disagree/2 = disagree/3 = somewhat agree/4 = agree/5 = completely agree
	Overall liking score for CC		10-point Visual Analogue Scale: 1 = lowest possible liking/10 = highest possible liking

¹α = Cronbach’s Alpha.

²This instrument originally consists of 17 items assigned to knowledge, coping and skills for hospital consultations. However, one item about expecting to be ready for the transfer to adult care was deleted, because it did not apply to our full sample.

³Young persons in phases 0 and 1 are still fully dependent on adults, e.g. parents, or display typical child behaviour. Young persons in phase 2 experiment with adult behaviour or orient to it. Phase 3 refers to full autonomy in participation. Because we were interested in successful transition to adulthood, the phases were dichotomised as follows: 0 = phases 0–2, 1 = phase 3.

⁴Construct validity was established in a previous study [45].