People familiar with the development of qualitative methods know that process tracing has developed rapidly over the last years. As the discussion about a method becomes broader and deeper, it becomes more important to systematize and sort the field in order to understand what it can and cannot achieve. Beach and Pedersen (BP) have left a mark on the field with their comprehensive book on process tracing and argue, among other things, that there are three varieties of process tracing. In my view, this is incorrect and ignores an important variant of process tracing that can be done and has been implemented a great many times in the empirical literature.
BP distinguish between case-oriented and theory-oriented process tracing, which is a useful distinction (and corresponds to case-centered and theory-oriented research in my book). Case-oriented research aims to develop a comprehensive explanation for one substantively interesting case. The two theory-oriented variants are theory-testing and theory-building. What theory-testing process tracing is need not be addressed in more detail here. The problem lies in the idea of theory-building process tracing which is taken as equivalent to exploratory research. It is common to treat theory-building research and exploratory research as being synonymous, but they are not because there are two variants of exploratory research.
We need to distinguish between process tracing that builds theory or, on a smaller scale, a hypothesis from scratch and process tracing that modifies an existing hypothesis. The latter variant is commonly referred to as puzzle solving and is one of the classic means of motivating empirical research. By and large, a hypothesis works but there are some cases that it fails to explain and that represent the puzzle one aims to resolve by modifying it. In contrast, genuine hypothesis-building research addresses a heretofore ignored or new phenomenon such as a country’s complete abandonment of nuclear plants or policy-making about pre-implantation diagnostics. We do not solve a puzzle here because there is no theory in light of which the new phenomenon could constitute a puzzle.
One might wonder what the big deal is about the fourth variant because these seem to be subtleties concerning ways in which to systematize process tracing. However, there is more to this because process tracing aiming to build and modify a hypothesis is built on different types of cases that call for different case selection strategies (see chapter 3 of my book). This can be easily demonstrated by bringing classic types of cases to the table. Two types of cases that are considered particularly interesting for exploratory research are failed most-likely and passed least-likely cases (related to theory-infirming and theory-confirming cases in Lijphart’s seminal 1971 article). A failed most-likely case does not pass a test it should have passed; a passed least-likely case masters a test we did not expect to be successful.
It is apparent that these two types serve to modify a hypothesis (some strategies of hypothesis modification are described in chapter 4 of my book). It is equally apparent that genuine exploratory research concerned with new empirical phenomena cannot be a failed most-likely case or a passed most-likely case because there is no hypothesis we can test in the first place (a similar argument holds for deviant cases). The two types of cases we can use are typical cases and diverse cases, as they only require information about the outcome, which is usually available in exploratory research.
The distinction between case-oriented and theory-testing process tracing, and confirmatory and exploratory process tracing thus is not all that we need to derive all possible types. Doing justice to exploratory process tracing and its proper implementation additionally demands it to distinguish between the analysis of new phenomena and the modification of an existing hypothesis that works well, but not perfectly.