rarefying

Rarefying is the process of making something rarer or less dense, and as a technical term it is used in multiple scientific fields with related but distinct meanings. In physics and engineering, rarefying refers to the expansion or thinning of a gas, producing a region with lower density and pressure. In rarefied gas dynamics, the mean free path becomes comparable to system dimensions, and continuum fluid models cease to be accurate. In acoustics, a rarefaction is the portion of a longitudinal wave where particles are spread apart; rarefying describes the accompanying decrease in density.

In ecology and biodiversity research, rarefying (rarefaction) is a statistical method to standardize sampling effort by subsampling data to a common sample size. This allows comparisons of species richness or other diversity metrics across samples that differ in size or sequencing depth. Rarefaction curves depict the expected number of species as a function of sampling effort. Critics note that rarefaction discards data and can reduce statistical power; alternative approaches include modeling-based methods that adjust for uneven sampling.

In microbiology and metagenomics, rarefying sequencing depth is routinely applied to equalize read counts across samples before downstream analyses. Ongoing methodological debates emphasize preserving information with alternative normalization strategies, though rarefaction remains a common default in some workflows.

Etymology derives from rarefaction, the act of becoming rarer, from Latin rarus.