HISTOGRAMS AND FREQUENCY POLYGONS

Drawing Histograms Bar graphs - discrete data Histograms - continuous data The ‘bars’ (or columns) are joined together. The horizontal (x) axis will always be a number line.

This histogram represents the exam marks of NCV 3 students at the end of the year…

Activity 7 Groups Midpoint X Frequency f 1,50 ≤ x < 1,55 1,53 2 1,58 3 1,60 ≤ x < 1,65 1,63 5 1,65 ≤ x < 1,70 1,68 7 1,70 ≤ x < 1,75 1,73 6 1,75 ≤ x < 1,80 1,78 1,80 ≤ x < 1,85 1,83 4 1,85 ≤ x < 1,90 1,88 1,90 ≤ x < 1,95 1,93 1 n = 35

Activity 6 (continued) 3. Mean = 1,71 m (to 2 decimal places) 4. Median lies in interval 1,70 ≤ x < 1,75 Therefore median is approx 1,73 metres Modal class is 1,65 ≤ x < 1,70

Frequency Polygons Representation of data linked to a histogram. Constructed by plotting the middle point of each class interval. The midpoints are then joined by straight lines to form a polygon. It is important to include an extra interval to the left and to the right of the required intervals to close the polygon.

It is not necessary to first draw a histogram before drawing a frequency polygon. Insert a class interval at the beginning and the end of the frequency table with a frequency of zero Find the mid points of the class intervals Plot the frequencies for each midpoint Join the points with straight lines to form the polygon.

Different shapes of Histograms Almost symmetrical distribution Speed of mammals

Skewed distribution Time intervals between nerve pulses

A double histogram