Question

indicate whether the sample is biased or unbiased and explain your choice using your own words and in complete sentences. In some cases, it may be appropriate to answer "not sure" because there is insufficient information to make a judgement. In these cases, if any occur, describe any additional information that you would like to have before deciding whether is biased or unbiased. In each of these excerpts, it is important to realize that you cannot assume that a sample was drawn as a simple random sample unless the researcher explicitly states that this was done.  

Excerpt 1: Researchers state, "A sample of 60 was randomly selected from 125 people arrested on DWI offenses and referred by the county to a professional evaluator between May 2000 and December 2003.

Excerpt 2: Researchers state, "To help resolve the questions posed in our research report, a telephone survey of 1000 California residents was conducted ... using random digit dialing (RDD). Sampling by means of RDD avoids the threat of systematically excluding that portion of the population with unlisted telephone numbers.

Excerpt 3:   Researchers state, "Nine counseling sites were invited to participate in the study. These sites were selected to provide a representative range of the types of locations at which counseling takes place. The locations were a university counseling center, a psychology training clinic, a women's center, a Lutheran Social Service center, two community mental health centers, and three private psychology practices. Two of the private practice sites declined to participate.

Answer 1

Excerpt 1: Researchers state, "A sample of 60 was randomly selected from 125 people arrested on DWI offenses and referred by the county to a professional evaluator between May 2000 and December 2003.

Answer: This is an unbiased sample because 60 people were randomly selected from the population of 125.

Excerpt 2: Researchers state, "To help resolve the questions posed in our research report, a telephone survey of 1000 California residents was conducted ... using random digit dialing (RDD). Sampling by means of RDD avoids the threat of systematically excluding that portion of the population with unlisted telephone numbers.

Answer: This is a biased sample because residents of California who did not have their telephone numbers listed were not considered.

Excerpt 3:   Researchers state, "Nine counseling sites were invited to participate in the study. These sites were selected to provide a representative range of the types of locations at which counseling takes place. The locations were a university counseling center, a psychology training clinic, a women's center, a Lutheran Social Service center, two community mental health centers, and three private psychology practices. Two of the private practice sites declined to participate.

Answer: This is a biased sample because counselling sites were intentionally selected.