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Graphing Data
Use a graph when you wish to:
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see overall trends, patterns or relationships in the data,
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compare two or more factors in a general or quantitative fashion,
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present large data sets in a comprehensible way and
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analyse data.
General Principles of Graph Construction
Towards a clear comprehension
1.
Transform observations into graphical form.
2.
Proofread your graphs (at least three times).
3.
Work towards clarity.
Evaluation Criteria
Evaluation criteria for graphs (include but are not limited to the following):
Presentation: Graph type, Data : ink ratio, Layout
Data: Symbols, Axes units, Axes labels
Caption: Description, Analysis (If applies), nomenclature
All graphs must be done by hand and on
millimetre paper.
Presentation
I- Plot type
When choosing the plot type one should consider whether the graph is appropriate for scientific
presentation and the way that the data were collected.
The simplicity of computers in plotting
will raise questions that one should address: the appropriateness of histograms vs. pie charts,
vs. the data points themselves.
Which is appropriate, when and under what circumstances?
Here are some examples of different graph or plot types:
Bar graphs (horizontal or vertical)
These are very simple graphs that consist of a number of proportional bars of equal width and
variable length.
Variations in quantity are scaled
along one axis only
(values may be scaled
on the x-axis for horizontal bar graphs and on the y-axis for vertical bar graphs).
Bars can
represent a wide range of variables, i.e. places, areas, items of different types and time periods.
They may be used on maps as alternatives to pie charts.
Their bulk and clumsiness restrict
their visual impact.
They allow quantitative information to be scaled against time, distance or
some other variable.
Bar graphs should not be confused with
histograms, which have
quantitative scale along both axes
.
Histograms
Histograms are similar to bar graphs in that they consist of a number of proportional bars of
equal width and variable length.
They differ in that
histograms are used to analyse and
study distributions
.
Also, histograms have a quantitative scale on both axes while bar graphs
have a quantitative scale on only one axis.
Typically, before data are plotted on a histogram,
the data range must first be divided into a number of intervals and the number of observations

falling into each interval recorded.
The percent of observations in each interval can be
calculated by dividing the number of observations in the interval by the total number of
observations and multiplying the resultant term by 100.
These results are then plotted with the

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