Graphical techniques

Graphical techniques

1. Histograms

   • Frequency of data is presented in classes.
   • Arbitrary choice of class intervals.
     • Stepped shape of histogram, then is usually smoothed out & replaced with a frequency distribution curve.
   • Curves may be normal with a symmetrical shape, or asymmetrical and skewed.
     • Often positively skewed, with the tail of the distribution extending to the right.

2. Bar charts

   • Series of rectangles, proportional in length and area to the values they represent.
   • Appropriate for presenting data that relate to discrete places, groups or time units.
     • eg. gender population structures.
   • Stacked bar charts represent two or more data sets by subdividing the bars.
     • May show absolute values, or values which are proportional (percentages).

3. Line charts

   • Appropriate where data is continuous rather than discrete.
     • eg. hydrograph showing river discharge over a month/year.

4. Pie charts

   • Circular graphs, subdivided into segments to show sub-groups in a population.
   • Total population is represented by the total area of the chart.
   • First segment normally starts at 0 degrees.
   • Number of segments is usually limited to 7 or 8.

5. Scatter charts

   • Used to plot two variables.
     • X is the independent variable - causes the change in Y.
     • Y is the dependent variable, plotted on the horizontal axis.
   • Provide a visual impression of the relationship between variables.
     • Closer the scatter of points, the stronger the relationship.
   • Positive relationship - points trend from bottom left to top right.
   • Negative relationship - points trend from top left to bottom right.