Longitudinal Classic Grounded Theory

A cross-sectional grounded theory study collects all data at one point in time and builds a theory about a social process as participants currently experience it. A longitudinal design returns to the same phenomenon — and often the same participants — at later time points. This allows the emerging theory to be tested against change: does the core category still explain behavior six months or two years later? The theory produced is explicitly temporal, capturing how the social process evolves rather than merely describing it at a single moment.

Classic GT does not prescribe fixed sample sizes; theoretical saturation guides sampling. In practice, longitudinal studies often begin with 10–20 participants and re-interview subsets based on theoretical sampling needs. Two to four waves separated by months or years are common; the number of waves should be driven by the rate of change in the phenomenon and by when the emerging theory reaches saturation across time points.

Attrition is an inherent challenge in longitudinal qualitative research. In classic GT, theoretical sampling means the researcher can seek new participants whose situations illuminate underdeveloped categories — so the departure of original participants, while a methodological challenge, can sometimes be addressed by sampling new informants who share relevant characteristics. The researcher must document how attrition affects the development of each category and be transparent about any gaps in coverage.

Classic GT recommends delaying an extensive literature review until after initial categories have emerged, to prevent the literature from prematurely constraining what is discovered in the data. This is not a prohibition on all background reading — familiarity with the substantive area is normal — but a discipline against coding the data through the lens of established frameworks before the data have had a chance to speak.

Memos must be maintained across all waves and should explicitly track how categories develop, stabilize, or change over time. A memo written after wave one becomes a reference point for wave two analysis: the researcher asks whether the same conceptual memo still holds, needs revision, or should be superseded. This temporal layering of memos is one of the most distinctive and analytically rich features of longitudinal grounded theory work.