Another method of representing a frequency distribution graphically is by means of histograms. In the frequency polygon all the scores within a given interval are represented by the midpoint of the class interval. But in the histogram, the scores are assumed to be spread uniformly over the entire interval.
In a histogram, frequency is shown by rectangle, the base of rectangle being the length of the class interval and the height is the corresponding frequency or the number of scores within the interval.
Histogram is the most common type of graph for displaying classified data. It is a bar or column graph with no space between the bars. It is also drawn on a pair of co-ordinate axes. The vertical scale or Y-axis is usually taken to begin at zero. The horizontal scale or X-axis can begin at any convenient number and one simple selects any convenient point to begin in it class- intervals. These class-intervals are represented in the X-axis with convenient units. The height in the Y-axis is determined by the number of observations for each class.
In drawing up histograms, we take into consideration the upper and lower limits of class-intervals. The upper limit of one class-interval becomes the lower limit of the next interval and as such there remains no space between rectangles of the histogram so drawn up.
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The histogram is said to be more exact than the frequency polygon. But a frequency polygon is more useful for comparison of two or more groups plotted on the same graph paper. More so, the frequency polygon can be formed by placing dots on the top midpoints of each rectangle of the histogram and then connecting the dots with straight lines. So after drawing the histogram, if it is so desired, frequency polygon can be drawn up.