noun
A group that is part of a larger group, often used in classification, statistics, or organizational contexts.
この研究では、参加者を年齢で三つの下位群に分けた。
In this study, the participants were divided into three subgroups by age.
サブグループ is a direct loanword for 'subgroup' and is more common in casual or technical contexts, whereas 下位群 is a Sino-Japanese term often used in formal or academic writing.
Compound of 下位 (kai, 'lower rank') and 群 (gun, 'group'). The exact historical derivation is uncertain, but the term is conventionally used to mean 'subgroup' in modern Japanese.