What is internal validity and why does it matter? What will be ...

[frankwu]

What is internal validity and why does it matter?
 
  What will be an ideal response?

Question 2

Which of the following is a good reason to choose a between-subjects design for an experiment?
 
  a. the researcher is able to recruit only a few participants for the experiment
  b. the researcher knows that there may be a practice effect as a result of repeated testing of the participants on the dependent variable
  c. the researcher expects the independent variable to have only a small effect on the dependent variable
  d. it is easier to do a between-subjects experiment

[parker125]

Answer to Question 1

Internal validity is the approximate truth about inferences regarding cause-effect or causal relationships. What does it really mean? Establishing internal validity is just a technical way of saying that you have ruled out plausible alternative explanations and have thus shown that a causal relationship likely exists between your program and the intended outcome. The key question in internal validity is whether observed changes can be attributed to your program or intervention (the cause) and not to other possible causes (described as alternative explanations for the outcome).
How do you go about establishing internal validity? One of the major ways is through the research design you apply. That is, a specific research design can address particular issues and help you eliminate the alternative explanations for your results, thereby enhancing the internal validity of your causal conclusions.

Answer to Question 2

B