Threats to EXTERNAL Validity
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Reactive Effect to Testing
PRE-TEST can cause a reaction in the subject so that their sensitivity is increased as a result; can increase/decrease post-test result; ALWAYS happens but try to minimize it. NOT THE SAME AS THE TESTING EFFECT!
Reactive Effect to the Experimental Arrangements
The SETTING of the experiment causes a reaction, therefore that data can't be extrapolated to the real world -- e.g. weight loss programs
Multiple Treatment Interference
The effect of a previous medical treatment has on a subsequent medical treatment; requires a WASH OUT period
Interaction of Selection Bias & Experimental Variable
Use of VOLUNTEERS for a group can result in that group having PARTICULAR BIAS or COMPOSITION