1. Census: a survey which measures an entire population.
Example: The United States conducts a census of the American population every ten years.
A census is very tedious and takes a long time to complete. Therefore, we use samples of a population to make inferences about that population.

2. Sample a small number of individuals representing a larger group.
Population
Sample

3. Population
Whether a researcher is drawing a sample or is studying an entire population, the population needs to be defined. This helps focus the research.
Example: If I want to know what type of candy 7th grade students prefer, then my population is defined as the set of all 7th grade students.
Population
Sample

4. Types of Sampling Simple Random Sample Stratified Random Sample
Cluster sampling
Systematic
Convenience
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5. Simple Random Sample
Every subset of a specified size n from the population has an equal chance of being selected
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6. Stratified Random Sample
The population is divided into two or more groups called strata, according to some criterion, such as geographic location, grade level, age, or income, and subsamples are randomly selected from each strata.
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7. Cluster Sample
The population is divided into subgroups (clusters) like families. A simple random sample is taken of the subgroups and then all members of the cluster selected are surveyed.
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8. Systematic Sample
Every kth member ( for example: every 10th person) is selected from a list of all population members.
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9. Convenience Sample
Selection of whichever individuals are easiest to reach
It is done at the “convenience” of the researcher
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10. Errors in Sampling Non-Observation Errors
Sampling error: naturally occurs
Coverage error: people sampled do not match the population of interest
Underrepresentation
Non-response: won’t or can’t participate
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11. Errors of Observation
Interview error- interaction between interviewer and person being surveyed
Respondent error: respondents have difficult time answering the question
Measurement error: inaccurate responses when person doesn’t understand question or poorly worded question
Errors in data collection
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