The Parameters of Mi'ut Hamatzui
A basic criterion for deciding many halachos is the concept of following the rov (majority of circumstances) versus mi’ut (minority). However, mi’d’rabanan (by rabbinic decree), mi’ut hamatzui (a minority occurring with frequency) must be taken into account. This is the reason we are required to inspect slaughtered animals for certain treifos (conditions that would render them non-kosher), clothes for sha’atnez, food for insects, and much more—even though these issues don’t present in the majority of cases.
The Rivash defines mi’ut hamatzui as a phenomenon that occurs close to half the time, although it can be debated whether he means exactly forty-nine percent, or anything occurring in more than one-quarter (i.e. twenty-six percent) of cases. A later authority, the Mishkenos Yaakov, maintains that even as infrequently as ten percent counts; the Tzemach Tzedek—among other poskim—says even less than ten percent, bringing proof for this position in their writings.
Another perspective on mi’ut hamatzui, brought by many poskim, doesn’t determine the frequency percentage-wise, but defines it as a regular (though sporadic) occurrence, as opposed to a chance occurrence. An additional viewpoint qualifies the common minority as something that “comes as no surprise.”