After folic acid supplementation became standard in the u. s., the frequency of neural tube defects dropped to an average of just 1 in 5,000 live births. the observed frequency in the experimental group of the smithells study was much higher than 1 in 5,000. select the two best explanations for why this would be. the observed frequency in the experimental group of the smithells study was much higher than 1 in 5,000. select the two best explanations for why this would be. the individuals in the study may have been predisposed to neural tube defects. the individuals in the study were randomly assigned to the control or experimental group. the sample size in the smithells study was small. the sample size in the smithells study was large.