问题如下：
What are the median and interquartile range? When is the median equal to the mean?
解释：
The median is the point where 50% of the probability is located on either side. It may not be unique in discrete distributions, although it is common to choose the smallest value where this condition is satisfied.
The IQR is the difference between the 25% and 75% quantiles. These are the points where 25% and 75% of the probability of the random variable lies to the left. This difference is a measure of spread that is like the standard deviation.
The median is equal to the mean in any symmetric distribution if the first moment exists (is finite).
symmetric distribution 可以理解为 normal distribution 吗？
