Find the name of the midpoint of a class-interval.
Hint: We first explain the difference between class limits and class boundaries. Then we take the average value of the class boundaries to find the class mark, the midpoint of a class-interval.
Complete step-by-step solution: The name of the midpoint of a class-interval is class-mark. For each class we have two class limits. They are called upper class limits and lower-class limits. The lower-class limit of a class is the smallest data value that can go into the class. The upper-class limit of a class is the largest data value that can go into the class. Then comes class boundaries and they are halfway points that separate the classes. The lower-class boundary of a given class is obtained by averaging the upper limit of the previous class and the lower limit of the given class. The upper-class boundary of a given class is obtained by averaging the upper limit of the class and the lower limit of the next class. Class marks are the midpoints of the classes and are obtained by averaging the boundaries.
Note: Through in a grouped data the class intervals can be of unequal size, usually we will meet class intervals of equal size. In case of non overlapping intervals, the gaps between consecutive intervals may be unequal. But at this level we will deal with intervals with equal gaps between consecutive classes.