According to AHRQ, all complex interventions have two common characteristics (Kelly et al. 2017):

• They have multiple components (intervention related)
• They have complicated/multiple causal pathways, feedback loops, synergies, and/or mediators and moderators of effect (pathway related)

The NIH has guidance on how to develop complex interventions and has developed a  framework of actions for intervention development (O’Cathain et al. 2019). While the framework is focused on developing complex interventions and not delivering them, some key advice pertains to both, including involving teams that include those who will deliver, use, and benefit from the intervention, and understanding context.

Similarly, PCORI has standards for studies of complex interventions (2015). The key aspects that inform intervention delivery include defining the casual pathway, which involves an understanding of contextual factors that may impact intervention delivery. PCORI requires recipients of its research funding to fulfill the following requirements when designing and conducting studies with complex interventions:

• Fully describe the intervention of interest and the comparator
• Define core functions for the intervention and the comparator
• Specify the hypothesized causal pathways and their theoretical basis
• Specify how adaptations to the form of the intervention and comparator will be allowed and recorded
• Plan and describe a process evaluation
• Select patient outcomes informed by the causal pathway

From the field of Dissemination and Implementation Science, The Practical, Robust, Implementation and Sustainability (PRISM) framework, also emphasizes how contextual factors can impact implementation and sustainability of an intervention (McCreight et al. 2019). Other resources include TIDieR framework, which refers to the types of location where the intervention will be delivered (Hoffmann et al. 2014), and the Context and Implementation of Complex Interventions (CICI) Framework, which describes contextual issues and provides guidance regarding how to understand studies according to their context (Pfadenhauer et al. 2017).

Another tool available for measuring intervention complexity is the Intervention Complexity Assessment Tool (iCAT-SR) (Lewin et al. 2017; Lewin et al.) for use in the context of systematic reviews. After a thorough review of the literature, the NIH Pragmatic Trials Collaboratory team used the iCAT-SR tool as a starting point for identifying key domains of complexity of intervention delivery.  This starting point eventually led to the factors included in the Intervention Delivery Complexity tool, described in the next section.