Bacterial species pangenomes, the set of all the species’ genes, are highly variable across taxa. Several recent studies have investigated correlations between pangenome diversity and effective population size (Ne), which represents the relative strength of selection vs. genetic drift, and is associated with differing bacterial lifestyles (e.g., endosymbionts vs. free-living). There is a clear positive association between these measures, and a recent causal analysis highlighted a best-fitting model with lifestyles, rather than Ne, directly impacting pangenome diversity. However, estimating Ne is challenging from bacterial genomic data, and the best approach remains an open question. We sought to explore the impact of different approaches for estimating Ne proxies on the conclusion that lifestyle traits best predict pangenome diversity. We are analyzing 928 bacterial genera, with > 70,000 genomes, for which we consistently computed pangenome annotations. Selection efficacy increases proportionally with Ne and is thus commonly used as a Ne proxy. We will evaluate a common metric for selection efficacy: the ratio of non-synonymous to synonymous substitution rates, both between and within species. We will also evaluate measures based on amino acid distances, as well as synonymous site diversity. We will use causal modelling to determine whether metric choices alter conclusions about the relative importance of lifestyle traits. Our results will have direct implications for understanding which evolutionary and ecological factors drive variation in bacterial gene content.