1- Measures of Central Tendency
Let's Suppose that we have a sample with 4 observations: 4, 1, 4, 3 
Mean
Is the sum of a set of numbers divided by the number of observations.
Median
is the middle point of a set of numbers (for odd numbered samples).
The mean of the middle two points (for even samples).
Mode -
Is the most frequently occurring number. Mode=4 (4 occurs most).
2-Measures of Variation
Range
The maximum value minus the minimum value in a set of numbers.
Range = 4-1 = 3.
Standard Deviation
Is The average distance a data point is away from the mean.
Note :
1. Standard deviation: compute the difference between each data point and the mean. Take the absolute value of each difference. Sum the absolute values. Divide this sum by the number of data points. Median: first arrange data points in increasing order.
Mean, Median, Mode, Range, and Standard Deviations are measurements in a sample (statistics) and can also be used to make inferences on a population.
