Why is the threat of subject attrition more problematic in ...

[Collmarie]

Why is the threat of subject attrition more problematic in longitudinal designs than in other types of descriptive research?
 
  a. Subjects are elderly.
  b. Data analysis is complex, and missing data points necessitate dropping the subject from the analysis.
  c. Data collection occurs over a much longer period of time.
  d. There is selection bias because of the study requirements.

Question 2

Why does subject attrition affect internal validity?
 
  a. A study in which the majority of the subjects die calls into question whether the treatment itself is safe.
  b. Subjects who drop out may differ from those who stay in the study, in terms of an important extraneous variable.
  c. If subjects drop out of the control group, and not the experimental group, it strongly implies that there is some benefit to participation of which the researchers may not be aware.
  d. Subject mortality may result in a sample that is so much smaller than anticipated that type II error may result.
  e. Type I error is almost guaranteed with very unequal sample sizes.

[sierramartinez]

Answer to Question 1

ANS: C
Longitudinal designs examine changes in the same subjects over time. They are expensive and require a long period of researcher and subject commitment. Measurement must be carefully planned and implemented, because the measures will be used repeatedly over time. There is often a bias in selection of subjects because of the requirement for a long-term commitment. In addition, loss of subjects (attritionsubject quits participating in the study) can be high and can decrease the validity of findings.

Answer to Question 2

ANS: B, C
The subject attrition threat is due to subjects who drop out of a study before completion. Participants' attrition becomes a threat when (1) those who drop out of a study are a different type of person from those who remain in the study or (2) there is a difference between the kinds of people who drop out of the experimental group and the people who drop out of the control or comparison group