Mathletes are confused: what are the traditional methods of variable sampling?

1. Unit Mean Method: A variable sampling method that estimates the overall value from the unit mean of the sample.

2. Difference estimation method: a variable sampling method to infer the difference between the overall recorded value and the audited value through the difference between the recorded value of the sample and the audited value, and then make an evaluation of the overall recorded value.

3. Ratio estimation method: through the ratio of the audit value and the sample record value to infer the overall audit value and the ratio of the record value, and then estimate the overall audit value and the original record value of the correctness of the evaluation of the variable sampling method.