Table I.

Factors related to the choice of how to count ART treatment cycles in the setting of observational research.

	All cycles are distinct	Only retrieval cycles are distinct
Examples of research situations	•   Questions where the total number of treatment cycles matter and/or for exposures that may differ by retrieval or transfer cycle (e.g. cost-effectiveness of a particular treatment; patient lifestyle behaviors that change over time) •   Desirable when patients would prioritize minimizing the total treatment commitment (retrievals and transfers)	•   Questions looking at exposures that affect the cohort of retrieved oocytes (e.g. ovulation induction regimens; impact of oocyte age on outcomes) •   Desirable when patients would prioritize minimizing the number of necessary retrievals
Implications for data management	•   Simpler from a data management perspective	•   Exaggerates the issue of informative clustering, as patients with better ovarian response will have fewer retrieval cycles but may have a similar number of transfers •   May have situations where a single transfer uses embryos from different retrieval cycles •   Can be complicated from a data management perspective, as retrieval and transfer cycles need to be linked •   Unclear how to manage multiple retrieval cycles (‘embryo banking’) prior to any embryo transfers
Implications for statistical analysis	•   Need to account for clustering among all cycles within each patient	•   Need to account for clustering among retrieval cycles within each patient
Implications for patient counseling	•   Gives patients a better sense of total treatment commitment by counting each retrieval and transfer cycle •   Assumes that patients care equally about the experience of retrieval and transfer cycles, which is likely not the case	•   Will typically underestimate the total treatment commitment by counting only a subset of cycles (e.g. retrievals) •   Assumes that patients are not impacted by transfer cycles, which is likely not the case