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98

CHAPTER 4

they do not like the professor, or they may respond

more

favorably if

they

want t”

help a

prof?ww thev

like.

Sampling Bias

In selecting a sample from a population there is always some degree of sampling error. This error is the discrepancy between the true value of a variable for the population and the value that is calculated from the sample, and it is expected and precisely estimated as part of sampling. A

different type of error is due

to

sampling bias, a type of sampling error

that is controlled or influenced by the researcher to result in misleading findings. Occasionally researchers will deliberately skew the sampling. The most obvious deliberate bias is selecting only those subjects that will

respond in a particular way

to

support a point or result. For instance, if a

researcher is measuring the values of college students and

wants to

show

that

the

students are concerned about helping others and being in-

volved in community service, bias would result if the researcher deliber- ately selected students in education or social work and ignored majors that might not be so altruistically oriented. Selecting friends or col-

leagues may also result in a biased sample. An even

more

flagrant type

of bias occurs when a researcher discards some subjects because they have not responded as planned or keeps adding subjects until the de- sired result is obtained. Sampling bias also occurs nondeliberately, often because of inadequate knowledge of what is required to obtain an unbi- ased sample and the motivation to “prove” a desired result or point of

or

view. In qualitative ful about possible study.

studies the researcher needs to be particularly care- unintended bias if sampling changes during the

Bias can also tions and assigning parison. Suppose a

result from selecting subjects from different popula- them to different groups for an experiment com- researcher used graduate sociology students to re-

ceive a treatment in an experiment and graduate psychology students as a control group. Even if the samples were selected randomly from each population, differences in the populations, and consequently samples, in attitudes, values, knowledge, and other variables could explain why certain results were obtained.

CONSUMER TIPS: CRITERIA FOR EVALUATING SUBJECTS SECTIONS OF REPORTS AND SAMPLING PROCEDURES

1. The subjects

in

tbe study should be clearly described, and the

description should be specific and detailed. Demographic

characteris-

Sampling bias: Sampling

caused bY the researcher.

error

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