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Internal Validity of True Experiements

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What are true experiments? How are threats to internal validity controlled by true experiments? How are they different from experimental designs?

What are quasi-experimental designs? Why are they important? How are they different from experimental designs?

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What are quasi-experimental designs? Why are they important?

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1. What are true experiments? How are threats to internal validity controlled by true experiments? How are they different from experimental designs?

By definition, all experiments involve manipulation of one or more independent variables, and observing the effect on some outcome (dependent variable). Experiments can be done in the field or in a laboratory. They can involve human or animal subjects. What distinguishes the type of experiment is the degree to which the experimenter can assign subjects to conditions. For example, three types of experiential designs include: True, Quasi- and Single-subject experiments.(http://psychology.ucdavis.edu/SommerB/sommerdemo/experiment/types.htm)

True experiments include the following components:

? Subjects are randomly assigned to the treatment conditions (levels of the independent variable). The only differences in the groups would be due to chance.
? Manipulation of one or more independent variables
? True experiments can show a cause-and-effect relationship.(http://psychology.ucdavis.edu/SommerB/sommerdemo/experiment/types.htm)

Internal Validity is "the extent that a research design and its data provide accurate views of relationships within the data - a cause-effect relationship" (http://74.125.113.132/search?q=cache:0Mh2jSKfcvgJ:facweb.cs.depaul.edu/mobasher/classes/csc426/notes/W4_Matthew-Goto_Internal-Validity.ppt+challenges+to+internal+validity&cd=1&hl=en&ct=clnk). Threats to internal validity are controlled through:

? Random assignment (or random assignment within matched groups) ...

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