This document discusses process tracing as a qualitative method for assessing causal inference within single cases. It explains that process tracing involves opening up the "black box" to understand the causal mechanisms between an intervention and outcome. The document provides an example of how process tracing was used to examine the impact of a universal health care campaign in Ghana. It describes how process tracing involves developing a causal mechanism, collecting evidence for each part of the mechanism, and using different tests to assess the evidence and determine the likelihood that the mechanism holds for that particular case.