# Statistics : Reminder

### Arithmetic mean

For any series(x_1, ..., x_n) of real numbers, we define its arithmetic mean by the formula : bar x= 1/n sum_(i=1)^n x_i
So to calculate the average of several values, we add all the values and divide the result by the number of values.

Using this formula, for example, we can calculate the quarterly average for a student who got 13; 16; 7 and 12 on his math homework.

### Weighted average

To calculate the weighted average of a statistical series defined by giving the pairs (x_i;n_i), where x_i is the value of the characteristic under study and n_i is the number of that value, i in NN, 1<=i<=p, we use the formula : bar x= (sum_(i=1)^n x_i*n_i)/(sum_(i=1)^n n_i)=(n_1*x_1+n_2*x_2+...+x_p)/(n_1+n_2+...+n_p)

Using this formula, it is for example possible to calculate the weighted average of a student who got 16 in French (coefficient 3), 11 in English (coefficient 2), 12 in mathematics (coefficient 5).

### The frequency

The frequency of a data is the quotient obtained by dividing the number of this data by the total number.

F="number"/"total number"