intervention. In practice, RCT designs are often more complex, with multiple study arms and different interventions or intervention levels. The schema proceeds by listing all interventions and arms, and then matching none, one, or multiple interventions to each arm. This structure closely follows _ClinicalTrials.gov_ ; to our knowledge, no metadata schema used in the social sciences provides this level of detail.<sup>9</sup> Contributors are first asked to list the study _interventions_ by assigning a short _name_ , classifying the _intervention type_ , and then providing an optional free text _description_ . The CV for the intervention type is still in development, as the CVs tested did not provide the right balance between broad classifications and detailed options for the kinds of interventions that occur frequently in social science RCTs. Note that the intervention type is one of three key fields that either alone or in combination help users narrow down the content of the study; the other two are the topic area (under I. Basic Information), and the outcome measures (see above). All three may be different: for example, Oster and Thornton (2012) randomize the distribution of menstrual cups in Nepal; they measure take-up by direct recipients as well as individuals in their social networks as the outcome; and the aim of the research is to understand peer effects in technology adoption. Finalizing the CVs for these fields will require testing with a larger body of studies. The new CVs will be posted on the GitHub repository for this project.

Next, contributors can select the _intervention assignment strategy_ from a CV and then optionally provide a more complete description. The CV options are adapted from _ClinicalTrials.gov_ and include parallel, factorial, and cross-over assignment, as well as the option “other”.<sup>10</sup> Contributors can also provide additional information, such as whether the random assignment was carried out using stratification. We encourage listing out all stratification variables. Stratification and other procedures aimed at improving balance such as re-randomization typically reduce variance but also need to be accounted for in the analysis.

Finally, the section records the _study arms_ . Contributors can give an identifying _name_ to each arm, list the _targeted_ and _actual number of randomization units_ in the arm, and then indicate which intervention(s) this group received (if any: a control group may receive no interventions). This allows users to back out, for example, how many subjects received the same intervention across treatment arms, and (for factorial designs) which intervention combinations are observed in the data.

Some experimental designs are common in the social sciences but less so in clinical trials. We make some recommendations on how to describe these designs using the options provided. One example is randomized phase-in, meaning that the intervention is randomly assigned to start at different times for different experimental arms, and the comparison between the arms (already) receiving the intervention and the arms

> 9For example, the AEA RCT Registry and YARD only have free text fields to describe the intervention(s) and do not between _interventions_ and _arms_ .

> 10Note that we dropped “single group assignment” and “sequential assignment” as these assignment strategies do not create a comparison group and therefore do not constitute an RCT by the common understanding of this term in the social sciences.

12