The idea

Frequency polygons

Turn a grouped frequency table into a frequency polygon (each frequency plotted at the midpoint of its class, the points joined with straight lines) and read one back into a table. Find the modal class, the class containing the median and an estimate of the mean from a polygon, compare two polygons on one grid in context, and spot the mistakes that cost marks.

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Statistics · Data representation

Frequency polygons

Why it works

Two tables, one question

Priya grows tomato seedlings in two trays: tray A in ordinary compost, tray B with a new plant food. After three weeks she measures the height,

h

cm, of all

40

seedlings in each tray. Did the plant food make them taller?

Height ( $h$ cm)	Tray A	Tray B
$0 < h \le 4$	$2$	$0$
$4 < h \le 8$	$8$	$5$
$8 < h \le 12$	$16$	$11$
$12 < h \le 16$	$10$	$14$
$16 < h \le 20$	$4$	$10$

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