Sampling Basics Jeremy Kees, Ph.D.

Conceptually defined… Sampling is the process of selecting units from a population of interest so that by studying the sample you can fairly generalize your results to the population from which the units were chosen

Review… Sampling is concerned with internal or external validity??

External Validity The degree to which the conclusions in our study would hold for other persons in other places and at other times

Threats to External Validity The population you want to generalize to is different from your sample The setting/place you want to generalize to is different from the place you conducted your study The time when you conducted your study was peculiar

How do we strengthen external validity?? Use random selection from the population of interest Replicate your study with different samples, in different settings, and over time

Procedure for Drawing a Sample Select the Sample Elements Define the Target Population Identify the Sampling Frame Select a Sampling Method Determine the Sample Size Collect the Data from the Designated Elements Step 1 Step 2 Step 4 Step 3 Step 5 Step 6

Terminology Population Parameter Sample Statistic Sampling Distribution Sampling Error (Sampling Error)

Defining the Population (Step 1) Who are we interested in generalizing to?

Define the Sample Frame In some cases, we can identify all possible cases and then randomly select our sample However, in social science research, this is not typically the case We do our best to chose a sampling frame that will give us our best shot and obtaining a sample that is representative of the population of interest

Select a Sampling Method So we have decided who we’re going to sample from, now we have to decide HOW to go about doing it

Sampling Methodologies: Probability Sampling Simple Random Sampling –Systematic Random Sampling Stratified Random Sampling Cluster Random Sampling Multi-Stage Sampling

Sampling Methodologies: Nonprobability Sampling Convenience Sampling Modal Instance Sampling Expert Sampling Quota Sampling Heterogeneity Sampling Snowball Sampling

Determine Sample Size Sample size is a function of… –Effect size –Alpha level –Statistical power Very difficult to estimate

Select Sample Elements / Collect Data Implement sampling method and collect the data

Quality Control Even professional marketing research firms need accountability We definitely need to have develop procedures to ensure the integrity of our procedures

Assignment #4 Develop a sampling plan 1.Discuss your target market and define the sampling frame 2.Select a sampling method and explain why it is most appropriate for your study 3.Determine your sample size 4.Explain your data collection strategy (e.g., how you would go about getting your sample to participate) Due March 2 (two weeks)