// *一种思路是全删、一种思路是每个都留下至少一个
    // bys auditor year: egen mnum=mean(num1)
    // bys auditor year: keep if num1==mnum
   //*或者
   //  bys auditor year: egen max_n=max(num1)
   //  bys auditor year: egen min_n=min(num1)
    // bys auditor year: keep if num1==max_n & num1==min_n


有大神帮我解释一下这组命令吗