Measures of Center
measures of central tendency:the center or most typical value:average
Mean:its arithmetic average；受极值影响；可以通过去掉极值减少极值的影响
Median:the middle value in its ordered list,Constructing a stem-and-leaf diagram as a preliminary step to ordering the data
is often helpful.
Mode(众数):Now, both the value 300 and the value 400 would occur with greatest frequency, 4.This new data set would thus have two modes, 300 and 400.与mean和median完全不一样，可以不在中间部分，. Of the mean, median, and mode, the mode is the only measure of center that can be used for qualitative data
Sum：
