Table 1.

Quantitative evaluation measures of the influence agents and their peers at T1 and/or T2

Measure name	Item(s)	Response options	Data sample	Time point
Enjoyment of the training	Did you like the training related to drinking water?	No, not at all No, not really Yes, a little bit Yes, a lot	Influence agents	T2
Duration of the training	What did you think of the duration of the training related to drinking water?	Too short Short Neither too short nor too long Long Too long	Influence agents	T2
Perceived autonomy support	I had the feeling that the researcher gave me choices. I felt understood by the researcher. The researcher showed that she had confidence in me to stimulate water drinking. The researcher encouraged me to ask questions. The researchers listened to how I wanted to stimulate water drinking. The researcher tried to understand my ideas before she herself came up with other ideas.	No, not at all No, not really Yes, a little bit Yes, a lot	Influence agents	T2
Intrinsic motivation	I drink water because … … I like it … I enjoy it … I think it is pleasant	No, certainly not No, I do not think so No, possibly not Yes, possibly Yes, I think so Yes, certainly	Influence agents and peers	T1 and T2
Water consumption	How many glasses of water did you drink yesterday?	Zero glasses per day One glass per day Two glasses per day Three glasses per day Four glasses per day Five glasses per day Six glasses per day Seven or more glasses per day	Influence agents	T1 and T2
SSB consumption	How many glasses of sweetened fruit juice did you drink yesterday? How many glasses of lemonade made of sugar syrup and water did you drink yesterday? How many glasses of soda did you drink yesterday? How many glasses of energy drink did you drink yesterday? How many glasses of sport drink did you drink yesterday?	Zero glasses per day One glass per day Two glasses per day Three glasses per day Four glasses per day Five glasses per day Six glasses per day Seven or more glasses per day	Influence agents	T1 and T2
Perceived social support	How often do your peers... … complement you on drinking water? … participate in drinking water with you? … remind you to drink water? … offer to drink water with you?	Never Rarely Sometimes Often Very often Always	Peers	T1 and T2
Descriptive norms	How often do your classmates drink water?	Never Rarely Sometimes Often Very often Always	Peers	T1 and T2
Injunctive norms	Do you experience that your classmates think you should drink water?	No, certainly not No, I do not think so No, possibly not Yes, possibly Yes, I think so Yes, certainly	Peers	T1 and T2
Water-promoting strategies	How often did you drink water when your classmates were with you? How often did you talk with your classmates about drinking water at school or home? How often did you talk with your classmates about drinking water on the social media platform (Social Buzz)? How often did you send videos about water drinking to your classmates on the social media platform (Social Buzz)?	Never Rarely Sometimes Often Very often Always	Influence agents	T2

Note. T1 = baseline; T2 = immediately after the start of the intervention