In order to investigate the teaching level of a school, the test scores of some first-year students in this school are randomly analyzed in order to fully reflect the reality.

Solution: (1) Among the three extraction methods, the whole is the test scores of all students in Grade One of our school, and the individual refers to the test scores of each student in Grade One of our school. The sample in the first extraction method is the test scores of 20 students in our school, and the sample size is 20; The second sampling method, the sample is the test scores of 20 students in this school year, and the sample size is 20; The third sampling method, the sample is the test scores of 100 students in this school year, and the sample size is100;

(2) Among the above three sampling methods, the first method is simple random sampling;

The second method is systematic sampling and simple random sampling.

The third method is stratified sampling and simple random sampling.

(3) The steps of the first sampling method are as follows:

The first step is to randomly select a class from these 20 classes by drawing lots;

Step two, by means of random number table or lottery, 20 students in this class are selected according to their student numbers, and their examination results are examined;

The steps of the second sampling method are as follows:

The first step, in the first class, a student with student number A is randomly selected by simple random sampling method;

Step 2, in the remaining 19 classes, choose students with the student number of A+NK (n = 1 2, …, 19, k is the number of students in each class), and there are 19 students;

Third, the individuals extracted in the first two steps constitute a sample;

The sampling steps of the third mode are as follows:

Step 1: Stratification, because if students are graded according to their grades, there are *** 150 excellent students, ***600 good students and ***250 average students, so when sampling, all students should be divided into three grades;

Step 2: Determine the number of people sampled at each level, because the ratio of sample size to total number of individuals is100:1000 =1:10, so the number of individuals sampled at each level is15,60,25 in turn;

The third step is to select 15 students from excellent students by simple random sampling. 60 students were selected by simple random sampling method among outstanding students; 25 students were selected by simple random sampling.