Contrast the following experimental designs with respect to factors affecting internal and external validity.?

Question

One Group Pretest - Posttest Non-equivalent Control Group Design

O1 X O2


O1 X O2 / O3 O4

WHAT AM I SUPPOSE TO DO TO ANSWER THIS PROBLEM? I DON'T UNDERSTAND. THANKS FOR YOUR HELP!

Answer 1

When you need to measure the effect of say, a new psycho-medicine on patients you need two groups of patients. One group will receive the medicine, while another group does not, the 2nd is the control group. The pretest group is tested before and after receiving the medicine. The posttest non-equivalent control group is the 2nd one randomly selected and tested after the event and no effort is made to match the two groups as to age, gender, length of time and extent of symptoms, etc. Thus the control group does not really match the test group and is not a valid comparison since the statistical analysis assumes that the two groups are samples from the same population.

You might be able to compare individuals within the test group (internally valid) but not with individuals in the control group (not externally valid) nor with individuals in any other similar facility (not externally valid).

The best design is where two similarly matched groups of patients are selected and one is given the treatment and the other receives a placebo. Neither the people who give the treatment, nor the people who carry out the tests know who is in which group. This avoids any effects from the behaviour of the people carrying out the experiment. Only when the final numbers are in are the identities of the patients and their group revealed and the statistical anaylsis carried out to determine the extent of any effect.

This would be called a matched (ie equivalent) groups double blind experimental design. The double blind refers to the fact that both the medicine givers and the test takers are blind to the group in which their patient falls. This design emerged because it was found that the behaviour of the experimenters (who were anticipating effects before the results were in) could affect the responses of the patients.

The results will be not only valid between the two groups (both internally and externally valid) but the results could be generalised to apply to individuals not in either group or even in different facilities (externally valid).

While lacking some specifics of your question I hope this gives you sufficient to carry out further research into the appropriate areas of test group formation in biologically oriented sampling and statistical methods, e.g. google 'double blind' and see where it takes you.

Answer 2

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Answer 3

You & me both mate -

I found the link below = could be some help ? (it contains the same mysterious symbols O x O)