Recruitment and Procedure
The target group were German-speaking parents of children aged 0 to 24 months who contacted a cry baby outpatient clinic in Bavaria (Southern Germany) for the first consultation because of crying, sleeping, or feeding problems. To avoid confounding the effects of app use with the effects of counseling, the individual study phase ended before the first appointment at the outpatient clinic (posttest [t2]). Participants whose possible study duration was very short (<10 days from first contact with the study team until counseling appointment in the outpatient clinic) were excluded because the applied questionnaires referred to a report period of at least 1 week. During the initial phone contact with the clinic, interested parents who met the inclusion criteria were referred to the study team. After verbal consent, study information, declaration of consent, and baseline test questionnaires (t1) were sent to their home addresses. As soon as the study team received the original signed informed consent and t1 questionnaires, families were included and randomly assigned to the IG or the WCG. Randomization was conducted by an independent researcher using Research Randomizer [