Quota sampling involves dividing a population into subgroups and selecting samples to represent each subgroup in proportion to its share of the overall population. It allows investigation of traits in subgroups and relationships between subgroups. While quick and cost-effective, it cannot calculate sampling error and may not represent the entire population. Snowball sampling relies on referrals from initial subjects to identify new subjects. It is useful for hidden populations but represents less control and potential sampling bias. Both methods limit statistical inference and introduce subjectivity.