Stratified sampling divides population into homogeneous strata, samples from each. Strata sizes: Proportionate (sample proportional to strata size) or Disproportionate (over/under sample for precision). Example: University 3000 students. Stratum A (science) 1500, Stratum B (arts) 1200, Stratum C (commerce) 300. Sample 300. Proportionate: 150 A, 120 B, 30 C. Disproportionate (to study C better): 100 A, 100 B, 100 C. Advantages: Ensures representation; reduces variance. Disadvantages: Need stratum information; more complex. Solving: Identify natural strata. Determine sample sizes. Randomly sample within each stratum. Exam tip: Use when population naturally stratified. Understand proportionate vs disproportionate. Practice: Stratified sampling design problems.