We employ a family-level randomized controlled trial with an opt-out design for parents of children aged three-to-five in five partnering municipalities in Denmark: Hoeje-Taastrup, Ikast-Brande, Langeland, Lejre, and Middelfart.
Recruitment: Each of the five municipalities provided us with a list of preschools that would participate in the evaluation; it was at the municipalities’ discretion to determine which preschools would participate, and the municipalities included between 67% and 100% of public preschools.
Sample: In total, 3,804 children born in or later than year 2016 (age 3 at the start of the intervention) were enrolled in the 70 participating preschools on October 1, 2019; these children constitute our population. Children in the population were included in the trial if two conditions were met on January 1st, 2020: First, that their parents had not opted out of the trial, second, that we had received at least one parental phone number from the collaborating municipalities. In total, 3,498 children in 3,181 families were included in the trial. At the start of the intervention their age distribution was as follows: 1,024 3-year-old children, 1,106 4-year-old children, 1,219 5-year-old children, and 149 6-year-old children.
Timing: Baseline language assessments were performed by the collaborating preschools in the Fall of 2019. Randomization was conducted in January 2020. On January 24, 2020, parents in the treatment condition received the first text message. Children in the control condition received treatment as usual. The last text message will be sent out on September 5, 2020.
Endline data: In September 2020, the municipalities will start collecting the first round of child-level endline data on language development (using “Sprogvurdering 3-6”). The data collection is scheduled to be complete by the end of 2020, at which time we will receive the data. In September-November 2021 and 2022, respectively, the municipalities will collect the second and third rounds of child-level endline data on language development (also using “Sprogvurdering 3-6”).
Information provided to parents: All parents in the participating preschools received the same informational pamphlet about the intervention in October 2019 before any data was collected. The pamphlet describes the intervention, the evaluation (including randomization), the data collection, and the research team. The information was delivered in printed form to the parents in the wardrobe of each preschool and was uploaded to most preschools’ intranet. The material was translated into eight languages; all parents received the printed version in Danish, and for each child the personnel in the preschool decided whether to attach a translated version. The material provided the parents with a link to a website that gave additional information, and with a phone number/email they could contact if they wished to opt out of the trial. In total, parents of 52 children opted out of the trial before randomization was conducted.
Spillover/contamination risk: To minimize spillover and contamination of the intervention to the control group, we aimed to minimize the probability that the preschool staff knew the allocation of children into the treatment or control group. We used three strategies to achieve this. First, the preschool staff was given no information about the allocation. Second, the preschool staff was explicitly instructed not to initiate conversations with the parents about the intervention. Third, to minimize the risk of parents engaging the preschool staff in conversation about the intervention, the informational pamphlet does not mention the preschool. Instead, the sender of the informational pamphlet is the municipality and the ROCKWOOL Foundation Interventions Unit.