In order to control for experimenter as well as confounding variables such as severity of condition, attitudes, age, etc., researchers assign subjects to groups by random assignment.

The answer to this question is the random assignment to control experimeter BIAS. This is the method or the only way in order to control for experimeter BIAS as well as confounding variables such as the severity condition, the attitudes, the age and etc., The researchers or the person who conduct subjects to groups by random assignment.

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How is it possible that phylogenies based on sequences from nuclear genomes and organellar genomes (i.e., chloroplasts and mitochondria) from the same group of species yield phylogenetic trees with different branching orders?   answer :   the genome of the endosymbiont is more closely related to members of the group in which it originally evolved, while the nuclear genome of the engulfing organism has its own evolutionary trajectory.