when designing an experiment why is it necessary to have multiple groups run in parallel? why should experiments be run in parallel why is it necessary to have multiple groups run in parallel why is it necessary to run an experiment with multiple groups in parallel why are parallel experiments necessary why is it necessary to have multiple experiments run in parallel why is it necessary to have multiple groups run in parallel? why would we need to run multiple experiments in parallel why should an experiment in parallel be done why do we need to test several experiments in a parallel why is a parallel research study necessary why is a parallel experiment necessary why is it necessary to have multiple experiments in parallel why is it necessary to have multiple groups run in parallel? what is the problem if the experiment is run in parallel with the control group why are parallel experiments necessary why is it necessary to have two groups run in parallel for a test? why is it necessary to run an experiment in parallel does the confounder cost to run an experiment why is it necessary to have multiple groups run in parallel