Five Number Summary
Consists of
 Minimum: the lowest point
 First quartile: the ¼ point in data
 Median: the center of data
 Third quartile: the ¾ point in dat.
 Maximum: the largest point
If a sample has even length, then the median is average of the two middle points:
 $\text{median} = \cfrac{a_{n/2} + a_{n/2 + 1}}{2}$
Same applies to 1st and 3rd quartiles
In R, command summary
returns the 5 number summary as well as the mean
Visualization
A visual summary of all the 5 numbers is called a box plot
Modified box plot
 is a variation of the box plot
 it's used to explain data with unusual values  Outliers

Measures of the Center
 Median is a measure of the center.
 But there is another measure  Mean or average value
 $\text{mean} = \cfrac{1}{n} \sum x_i$
 Where $n$  number of data values, and $x_i$  each data value.
The Spread
Mean and median don't show how spread the data is. There is another measure that address it: Variance.
 $\text{var}(x) = \cfrac{1}{n  1} \sum (x_i  \bar{x})^2$
 $s(x) = \text{std}(x) = \sqrt{\text{var}(x)}$
 ($n  1$ gives "unbiased" estimate of the variance TODO: add link)
in R:
st.dev = sd(data)
Sources