Making assumptions about individuals from group data is generally only reasonably safe when the variation within the group is small.

只有当一组数据内部变化程度相对较小时，据此做出的对个体的判断才会比较可靠。

youdao

It too sounds a fine idea, but even top private schools, with lots of money and good teachers, find it tricky to timetable lots of variation within one age-group's lessons.

ECONOMIST: Exams

There is tremendous variation in the prevalence and availability of fake drugs within and across countries, according to Kubic, whose group tracks counterfeiting incidents.

CNN: Fighting the growing menace of fake drugs