Читать книгу Practical Field Ecology - C. Philip Wheater - Страница 63

We need a variety of techniques to describe the data that we collect. This might be as a data exploratory technique (to check the data to see how variable a data set is, or what sort of distribution we get, etc.), to understand some aspects of the data (e.g. how diverse communities are), and for communication purposes (to be able to discuss the results, orally and in writing, with other people).

Here, simple plotting of measured variables on frequency histograms (or tables), cross‐tabulation of one (nominal) variable against another, and examining the range of the data (from the minimum to the maximum) may help to check for errors and ensure that we can chose the correct type of test for subsequent analysis. Extracting statistics, such as diversity indices to describe species richness, or evenness to describe how equal the proportion of species is within a community, can be important to assess what sort of community we have. Likewise, the estimation of population size or density might also be important. Similarly, we can use the average value of a variable to describe the magnitude of the majority of the data points (usually in conjunction with some measure of how variable the values are and how many data points were collected). These descriptive statistical techniques are reviewed later and more detail can be found in Wheater and Cook (2000, 2015).

Table 1.3 Common statistical tests. Note that in each case, there are possible questions (and analyses) dealing with more than two samples and/or variables – see Chapter 5 for further details.

Example question	Null hypothesis	Type of test	Data required
Is there a difference between the number of birds found in deciduous woodlands and coniferous woodlands?	There is no significant difference between the number of birds in deciduous and coniferous woodlands.	Difference tests, e.g. a t test or a Mann–Whitney U test (p. 305).	Two variables: one nominal describing the woodland type and one based on either measurements (i.e. actual numbers) or on a ranked scale that describes the number of birds.
Is there a relationship between the number of birds and the size of the woodland?	There is no significant relationship between the number of birds and the size of the woodland.	Relationship tests, e.g. correlation analysis (p. 307).	Two variables: one (either measured or ranked) that describes the number of birds and one (either measured or ranked) that describes the size of the woodland.
Is there an association between whether birds are resident or not and whether the woodlands are deciduous or coniferous?	There is no significant association between the frequency of residency and the frequency of woodland type.	Frequency analysis, e.g. a Chi‐square test (p. 312).	Two variables: one nominal describing the residency status of the birds and one nominal describing the woodland type.