You are conducting a survey of year$-12$ student performances in a senior high

school $($year$-10,$ year$-11$ and year$-12).$ Year$-12$ students in the school consist of

$50$ students entering the school since their year$-10($native students$),30$

students entering the school since their year$-11($long$-$term exchange students$)$

and $10$ students just entering the school for their year$-12($recent exchange

students$).$ To ensure you enough representatives from these $3$ groups of

students, you decided to interview $4$ students from each group. You then put

those students in your sample list.

During the interview $50\%$ of long$-$term exchange students and $25\%$ of recent

exchange students in your sample list did not come, you cannot interview them

and you did not do any replacement.

Q$1$: So that your survey result can be interpolated to the original population, you

need to assign weight to each group of students. Why?

Q$2$: Calculate the weight for each group in your survey result.