In a cumulative frequency graph (or histogram), the data is often given in class intervals. To calculate the estimated mean of the data, the formula is:
$$\frac{\sum m\cdot f}{\sum f}$$
where $m =$ midpoint of the class intervals and $f =$ frequency.
However, the main point of this question is, how do I calculate the midpoint of a class interval?
Example $1$
For a class interval of $30 \le x < 35$, the midpoint is $32.5$.
Example $2$
For a class interval of $0.1 \le x \le 0.5$ , the midpoint is $0.25$.
Example $3$
$x< 40\dotsc$ $x < 50\dotsc$ the midpoint between the two is $45$.
Example $4$
$< 10\dotsc < 20\dotsc < 35$ the midpoint is $4.5$, $14.5$ etc.
Is there a formula to this or something? I honestly don't get how to calculate the midpoints. Maybe it has something to do with the inequality signs?
Thanks.
Best Answer
The formula for a midpoint of a class interval is the lowest range plus the highest range divided by two. So say you have a class interval of $5-9.9$. You would calculate it this way:
$5+9.9= 14.9/2 = 7.45$ or approximately $7.5$