Unbiased and Biased samples

Simple random samples
Each students name is written on a piece of paper. The names are placed in a bowl, and names are picked without looking
Stratified Random sample
Students are picked at random from each grade level at school.
Systematic Random sample
From alphabetical list of all students attending a school, every 20th person is chosen.
Convenience Sample
To represent all the students attending a school, the principal surveys the students in one math class.
Voluntary Response Sample
Students at a school who wish to express their opinion are asked to the office after school.