18. The birth weights of 50,000 babies were recorded and analyzed. The distribution of the data is normal. Which portion of the data is less than the mean?
Answer: A
The portion of the data that is less than the mean is 50%.
In a normal distribution, the mean divides the data into two equal halves. Therefore, 50% of the data falls below the mean.
A) 50%
This option is correct because, in a normal distribution, exactly half of the values lie below the mean. This characteristic is fundamental to the definition of a normal distribution.
B) 68%
This option is incorrect. While 68% of the data falls within one standard deviation of the mean in a normal distribution, it does not indicate the portion below the mean specifically.
C) 95%
This option is incorrect. The 95% figure refers to the portion of data that falls within two standard deviations of the mean, not the portion below the mean.
D) 99.70%
This option is incorrect. The 99.70% figure represents the data that lies within three standard deviations of the mean, which does not pertain to the specific question of how much data is below the mean.
Conclusion
The correct answer of 50% is definitive as it directly corresponds to the properties of a normal distribution, where the mean splits the data into two equal parts. All other options refer to different aspects of the distribution and do not accurately represent the portion of data less than the mean.