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Lecture 4: Logic of Sampling

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Katie Stuart- Lahman

SOCB05 – Lecture 4 – May 30 2013 Logic of Sampling Who do you want to generalize to? – Theoretical Population What population can you get access to? – Study Population How can you get access to them? – Sampling Frame Who is in your study? – Sample. Research Project  Things to consider: o Can I study everyone? o How do I know who to study? o How do I pick my sample? o Will it represent everyone? Does it need to? o What limitations can I put on my sample? Why Sample?  We need a process to select participants/observations on which to carry out a study  Studying a whole population may be cumbersome/impossible. o Need a small group that approximates the population Two Types of Samping  Probability sample o was born in the 1940s, and remains the most common for gaining large representative samples.  Not always the best because some topics that are not suitable or appropriate. o Most common in social research, particularly quantitative o Representative of larger population. Elements of a population stand equal chance of selection o More complex.  Non-Probability Sampling o Common in qualitative studies o Vulnerable/hard to reach populations? o May be unrepresentative. Elements of the population do not stand equal chance of selection o Usually used in complex phenomenon that are not seeking generalizations. o Used on groups that are vulnerable, or hard to reach.  Example: Abused children, prostitutes, etc. People that there isn’t really a list of people fitting the category. Methods of Non-Probability Sampling  Haphazard Sampling o Aka “reliance on available subjects” o Cheap, convenient, and easier to find. o Use of those available at a particular place and time. o Useful for pretesting questionnaires or other social measurements o Not representative of a larger population  Difficult to generalize o Example: Picking people who walk past a store right near an entrance where the public transit is located.  Puposive/Judgemental Sampling o Sample selected on the basis of your own knowledge of the population you intend to study and your research question o Useful when studying a small subset of a larger population, where members of the subpopulation are easily identified.  Snowball Sampling o Already recruited respondents provide the researchers with assistance in locating other members of the population under study. o Useful when members of a population are particularly hard to locate  Example: Homeless people. o Exploratory purposes, as an introduction into the field. o Representativeness is problematic  People tend to refer people that are like them. So there will be a bias because the participants will be similar to each other. o Usually starts with one connection, who will share others that they know of. And it just keeps going from there.  Quota Sampling o The non-probability version of probability sampling. o Begins with a table describing the characteristics of the target population. o Try to assign equal proportions of people who belong to different groups to your sample. o Units are selected into the sample based on a specified list of predetermined criteria. Methods of Probability Sampling  Used when researchers want precise, statistical descriptions of large populations.  “to provide useful descriptions of the total population, a sample of individuals from a population must contain essentially the same variations that exist in the population” (167)  Representativeness= the quality of a sample of having the same distribution of characteristics as the population from which it was selected o Provides generalizability  When its not representative, it introduces bias:  Not using a probability sampling method  Having an inadequate sampling frame  Non-response bias. o Allows for use of inferential statistics  E.P.S.E.M o Equal Probability of Selection Method: When everyone has an equal chance of being selected.  Population: Theoretically specified aggregation of the elements in a study. Basically, all of the people that you are interested in. If you could study everyone, who would be included in your study?  Sample: Portion of the population from which information is colle
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