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Graphs Index

 Scattergrams   The relationship between oestradiol and testosterone levels during the menstrual cycle   These graphs are useful in trying to determine the relationship between two variables, as in the example above, the relationship between testosterone and oestradiol in women. These data are not necessarily the result of controlled experimental work but data from surveys or field observations can be used. If the relationship is linear (the points follow roughly a straight line) then the line of best fit or trend line can be drawn on the graph. The distance or spread of the points either side of this line indicates how close the two variables are related to one another (correlated). If the line rise to the right then there is said to be a positive correlation as in the case of the following example. If the line runs in the opposite direction, as shown below, then we say that there is a negative correlation between these variables.   The relationship between water temperature and the density of the flatworm Crenobia alpina in a stream, the Cotswolds G.B.

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Other Types of Graphical Representation

Line graphs or curves represent the most commonly used type of graphical analysis but there are others.

Histograms

These consist of a series of columns representing a variation in frequency (the number of times an event occurs) of a variable over a discrete interval or class. The example given shows the frequency of flowers in the flower heads of a sample taken from a population of red clover. It is worth noting that the area beneath the histogram is proportional to the frequency of the items being recorded. This is more easily demonstrated on graph paper where you can count up the squares under the columns of the histogram.

The distribution of the number of flowers per flower head in a population of red clover Trifolium repens

Bar Charts

These are very like histograms except that the columns are not usually adjacent to one another. This is to emphasise the point that these data are not directly related to one another. They could be placed in any order.

The effect of different mineral solutions on the germination of lettuce seeds after two days

Pie Charts

These graphs look completely different from any other form of graph so far considered. They are used to represent the proportions of a whole, as you can see in the example give. The proportions are based upon the size of the segment that each component occupies out of the whole disk (3.6° representing 1%).

Area Diagrams

Alternatively if the proportions are part of a series they may be expressed in a linear fashion as shown below.

The relative proportions of invertebrates caught in pitfall traps along a line transect
from a field into a woodland

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