Answer: the differences between experimental and control groups occur only by chance.
Step-by-step explanation:
In an experiment, the result is said to be statistically significant or to have statistical significance, if it is likely not caused by chance for a given statistical significance level. So, if an experimenter conducts an experiment on the effects of a drug to control hallucinations. he declares the results to be "statistically significant" which usually means that the differences between experimental and control groups occur only by chance