The intensive version of the program (NEP-I) was not part of the set of services originally offered by the Ministry of Health and was developed especially for this study. In spite of that, it was adopted by the Ministry of Health staff administrating NEP and also delivered at scale during the evaluation period. NEP-I was a collaborative effort between the Ministry of Health and a team of child development experts at Pontificia Universidad Catolica (working on the program Juguemos con Nuestros Hijos). It adds to the standard group intervention two practical sessions, where caregivers are given the opportunity to interact with their child and receive more personalized feedback. The rationale for the intensive version is to test the value added of offering opportunities for practical demonstration and skill building, which has been shown elsewhere to be associated with effectiveness in parenting interventions (Engle et al. 2007). The two added sessions focus on the importance of age-appropriate responsive play (reading children’s cues and providing scaffolding, through practice and discussion videos on sensitive play parent-child interactions) and on the importance of language and reading (through dialogic reading).

NEP-B has been fully scaled-up at the national level and is potentially highly cost-effective. It uses the infrastructure and human resources already existing in the health network with no further monetary and organization costs beyond training and material printing. The costs of NEP-B per family attended are only 10% of the costs of home visits. The more intensive version of the program being tested (NEP-I) costs 30% more than the standard version (NEP-B) per family attended.

# 2.2 The evaluation design

NEP was implemented across Chile. Therefore, our study is based on a representative sample of health clinics located in both urban and rural areas all over the country. The sample was stratified by type of clinic, which included family health centers, general health centers, and small hospitals (this stratification was motivated by the idea that different infrastructure and human resources across types of health centers may play an important role in the delivery of the program).

Within each clinic, a sample of 18 families was randomly drawn from a potential wait-list of participants formed by facilitators (which usually contains between 45 and 60 potential participants per center). Potentially eligible families<sup>8</sup> were identified during regular health visits to the center and added to the wait-list just prior to the administration of the baseline survey.

> 8 The recruiting procedure is as follows. Facilitators construct and review the waitlists checking the clinical records of each family, with the purpose of selecting those families that satisfied both the inclusion and exclusion criteria of eligibility for NEP. Families in crisis such as detected domestic violence, severe mental health problems, or child developmental delays that require clinical attention are excluded from the group sessions. Health professionals at the health center derive families in these cases to individualized attention. Once identified, parents of eligible families were invited to participate through home visits or through direct recommendation made by a health professional. Eligible families were enrolled after an interview with an NEP facilitator, where they were informed about this study’s intentions and about the randomized process of assignment to groups. Also, they were given the chance read the informed consent clause (or received an assisted reading of the same when they declared difficulties to read, poor reading skills or illiteracy). After accepting and signing the clause, they were included in a general database and the facilitators proceeded to randomly assign them to one of the groups. Parents who refused to participate continued receiving their usual health care offered at their usual health center and were encouraged to reconsider their entry for a future wave of NEP.

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