Design a study to evaluate the causal effect of a digital marketing campaign on sales using observational data.

First and foremost, clarifying the question is crucial. We are looking to understand the causal impact of a digital marketing campaign on sales. This implies that we need to isolate the effect of the campaign from all other variables that could influence sales. It's important to note that, given we are working with observational data, we won't have the luxury of randomized control trials (RCTs) which are the gold standard for establishing causality. Our challenge then is to approach this with methodologies suited for observational data while striving for the clarity and precision of an RCT. Data requirements play a critical role in this study...