• Who is in our study and how we sample them determines the generalizability of the results of our study. Poor sampling is a major threat to external validity.
  
  
• We have two types of sampling strategies: probability sampling and nonprobability sampling. Only probability sampling strategies result in representative samples.
  
  
• All probability sampling requires a sampling frame - a population defined and available through records.
  
  
• There are six probability sampling strategies typically used in psychological research: simple random sampling, systematic sampling, stratified sampling, proportionate sampling, cluster sampling, and multistage sampling.
  
  
• There are three nonprobability sampling strategies: haphazard, convenience, and purposive. Avoid haphazard samples at all costs.

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