Читать книгу Practical Field Ecology - C. Philip Wheater - Страница 59

 A population is a collection of individuals, normally defined by a given area at a given time. For example, scientists refer to the decline in the world population of Atlantic cod in the last century or the annual harvest of Northeast Atlantic cod. These are both true populations. The size of a population is rarely measured directly but usually estimated from samples.

 A sample is a term that can be used ambiguously, but is a subset drawn from a population, which usually includes a quantity. For example, 100 individual fish taken from the Northeast Atlantic cod population and measured in order to get an estimate of body size. Another example would be taking 50 small areas from a meadow (each 1 square metre in size) in order to count the number of plantains within them.

 A parameter is a population metric that is estimated from a variable (e.g. the mean body size of Northeast Atlantic cod, or the mean number of plantains per square metre of a meadow) and can be used to summarise data. Importantly, statistical tests aim to estimate parameters from a population in order to test for differences, relationships, associations, etc.

 A variable is a measurement that may change from sampling unit to sampling unit (e.g. the body size of Northeast Atlantic cod taken from a sample, or the number of plantains in a square metre of a meadow) and can be used to summarise collected data (e.g. by taking the mean).

The decision over which samples to take requires some care, and at this point it is worth discussing why replication is important. Since environmental systems are usually intrinsically variable (i.e. physical, chemical, and biological factors differ spatially and temporally), the larger the sample, then the more representative it will be of the population (i.e. the more of the natural variation will be covered). However, the larger the sample, the more time and effort it will take to collect it. There are methods to calculate the optimum sample size; however, these rely on knowledge of the variability of the system. This is rarely known in advance, although a small pilot study may give some indication. If it is known or suspected that there is substantial variability, then a large sample should be taken. In most ecological surveys, a large sample would include over 50 observations. However, where the population is likely to be very large and variation is expected to be great, even larger sample sizes may be required. Otherwise it is best to aim for as large a sample as possible after taking into account constraints including the size of the workforce, the time available, and how much material is present in the system under investigation. Sometimes, previous studies on similar topics can be used as a guide to what a reasonable sample size might be (i.e. from the literature or from a pilot study). Where several levels of a number of variables are to be analysed (e.g. male and female animals of each of three different age groups: young, mature, and old), then it is important to take sufficient replicates of each subgroup (e.g. young males, mature males, etc.) to be able to account for within‐group variability. This will inevitably have an impact on the required sample size and is another reason why the intended statistical analyses should be considered at an early stage of project planning. Box 1.7 shows the factors that should be considered when determining the sample size.