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Discussion in 'Mathematics' started by Newstein, Mar 5, 2016.
Should a frequency polygon for grouped data be closed or left open?
I would say left open
The book 'Taking the fear out of data analysis' (Pages 83 & 84)and some other textbooks say that by convention a frequency polygon is closed, by connecting the first point to the mid-class point of the (assumed) previous class on the horizontal axis, and the last point in a similar way. The word polygon really means a closed figure,doesn't it? I can't imagine an open triangle or open quadrilateral.
In terms of "should", there is no right answer.
It "can" be left open, or closed by joining to the assumed midpoint (or 0) as mentioned above. Students will often try to join the last point back to the first point which is wrong. In terms of exam marks, open is fine, and that is what I teach my students to do.
Remember the purpose is mainly for comparison with another fp. Does closing it help that comparison?