General:
* Controlled experiment
* Valid experiment
* Rigorous experiment
* Scientifically sound experiment
* Well-designed experiment
* Reliable experiment
* Accurate experiment
* Objective experiment
Focusing on specific aspects:
* Controlled conditions (emphasizing the importance of controlling variables)
* Replicated experiment (emphasizing the importance of repeating the experiment)
* Blind experiment (emphasizing the importance of eliminating bias)
* Double-blind experiment (emphasizing the importance of eliminating bias from both the experimenter and the participant)
The best synonym to use will depend on the specific context of your writing. For example, if you are emphasizing the importance of controlling variables, you might use the term "controlled experiment." If you are emphasizing the importance of repeating the experiment, you might use the term "replicated experiment."