The answer is A: Confounding.
Explanation for each option:
A. Confounding: This should not happen in experiments because it introduces a variable that can distort the results, making it difficult to determine the true effect of the independent variable on the dependent variable. Confounding variables can lead to incorrect conclusions and cannot be easily fixed once the data is collected.
B. Nonresponse rates: While high nonresponse rates can be problematic and lead to biased results, they can sometimes be addressed through statistical adjustments or by improving data collection methods in future experiments.
C. Double Blinding: This is a technique used to prevent bias in experiments, particularly in clinical trials. It ensures that neither the participants nor the experimenters know who is receiving the treatment or the placebo. Double blinding is a positive aspect of experimental design and should be implemented when possible.
D. Rigorous controlled experiment: This refers to a well-designed experiment with strict control over variables to ensure reliable and valid results. It is a desirable aspect of experimental design and should be aimed for in scientific research.
In summary, confounding is the issue that should not happen in experiments as it cannot be easily fixed and can significantly compromise the validity of the results.
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