The following are links to documents discussing sampling methods. There is a mix of simple and advanced (for the mathematically inclined) material. The target audience and context may vary, but the basic principles remain the same:
http://www.ssc.wisc.edu/~oliver/SOC357/Lectures%20and%20Notes/SamplingBigSlides.pdf
http://www.unesco.org/iiep/PDF/TR_Mods/Qu_Mod3.pdf
http://cphp.sph.unc.edu/PHRST5/IntroSampling.pdf
http://www.westfallteam.com/Papers/Sampling%20Methods.pdf
http://unstats.un.org/unsd/demographic/meetings/egm/Sampling_1203/docs/no_2.pdf
http://highered.mcgraw-hill.com/sites/dl/free/0073373605/582605/Chapter16_SamplingMethods.pdf
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