Frequency Polygon
The relevance of presentation of data in the pictorial or graphical form is immense. Frequency polygons give an idea about the shape of the data and the trends that a particular data set follows. Let us learn the step by step process of drawing a frequency polygon, with or without a histogram.
The key objective of statistics is to collect and organize data. One of the basics of data organization comes from presentation of data in a recognizable form so that it can be interpreted easily. You can organize data in the form of tables or you can present it pictorially.
Pictorial representation of data takes the form of bar charts, pie charts, histograms or frequency polygons. The benefit of this is that data in the visual form is easy to understand in one glance.
A frequency polygon is a graphical form of representation of data. It is used to depict the shape of the data and to depict trends. It is usually drawn with the help of a histogram but can be drawn without it as well. A histogram is a series of rectangular bars with no space between them and is used to represent frequency distributions.
· Mark the class intervals for each class on the horizontal axis. We will plot the frequency on the vertical axis.
· Calculate the classmark for each class interval. The formula for class mark is:
Classmark = (Upper limit + Lower limit) / 2
· Mark all the class marks on the horizontal axis. It is also known as the mid-value of every class.
· Corresponding to each class mark, plot the frequency as given to you. The height always depicts the frequency. Make sure that the frequency is plotted against the class mark and not the upper or lower limit of any class.
· Join all the plotted points using a line segment. The curve obtained will be kinked.
· This resulting curve is called the frequency polygon.
Note that the above method is used to draw a frequency polygon without drawing a histogram. You can also draw a histogram first by drawing rectangular bars against the given class intervals. After this, you must join the midpoints of the bars to obtain the frequency polygon. Remember that the bars will have no spaces between them in a histogram.