Читать книгу Practical Field Ecology - C. Philip Wheater - Страница 58

When implementing a project, it is rarely possible to collect information on all the animals or plants present. Usually we need to use a sample that we hope to be representative of the situation as a whole. The total number of data points that could theoretically be gathered is known as the population (this is a statistical population rather than the actual population of animals or plants – see Box 1.6); the actual number of data points is termed the sample size. Larger samples are usually more representative of populations, although this depends on the variability of the system being studied (small samples may be reliable representations of populations with low variability). Those elements of a system that are calculated (e.g. the mean number of plants, such as plantains, per square metre in a meadow) are termed statistics and are estimates of the true attributes of a statistical population (called parameters – see Box 1.6). So, if we counted all the plantains in the entire meadow, we would be able to calculate the actual mean value per square metre (a parameter). Since it is usually impractical to count all individual plantains, in reality we usually count plantains in a subset of the meadow (i.e. take a sample), and calculate the mean numbers per square metre using this sample in the expectation that it will be representative of the whole site (a statistic). This sort of situation occurs in many types of survey. For example, market researchers obtain opinions from large groups (samples) of people and use these to indicate the attitudes of the population as a whole.