Surveys and samples
Source: http://www.deakin.edu.au/~agoodman/sci101/chap7.html In this section you'll learn how sample surveys can be organised, and how samples can be chosen in such a way that they will give statistically reliable results. You will also see how, with a certain amount of knowledge of what is being surveyed, to decide on an appropriate sample size.
On completion of this section you should be able to:
− describe the relationship between censuses and samples
− list the sequences of stages involved in developing a sample survey − explain the formulation and operation of a sample frame
− define the major types of sampling methods, using both probability and nonprobability techniques
− describe the major forms of spatial sampling for selecting samples from phenomena that vary across the landscape
− explain the major factors that govern the calculation of minimum sample size, and the basic formulae for deriving sample sizes
Censuses and samples
In simple terms, you could call any data collection process that is not a controlled experiment a survey. Given the ethical problems associated with studying people (and the different issues in studying animals) much research in these fields is based on surveying rather than experimentation. In these terms a census is a survey whose domain is the characteristics of an entire population; a census is any study of the entire population of a particular set of 'objects'. This would include Eastern-barred bandicoots in western Victoria, human residents of Heidelberg, or the number of Epacris impressa plants on a single hillside in Garriwerd National Park. In each case there is a finite number of objects to study - although we may not know that number in advance and, as you'll see in a later section, we may need to estimate that number. On the other hand, we cannot in the same sense carry out a 'census' of the atmosphere or the soil.
If we are only able (or we choose) to collect, analyse or study only some members of a population then we are carrying out a survey. If the total population is finite and known, or continuous (infinite) then what we are doing is defining some proportion of the total population to study; we are therefore creating a sample.
Why do we use sample surveys? We have no choice when the population is continuous (that is, effectively infinite), but we can define a sample from either a finite or an infinite population. Surveys are done for several reasons. A sample survey costs less than a census of the equivalent population, assuming that relatively little time is required to establish the sample size. Whatever the sample size, there are 'establishment costs' associated with any survey. Once the survey has begun, the marginal costs associated with gathering more information, from more people, are proportional to the size of the sample. But remember that surveys aren't conducted simply because they are less expensive than a census: they are carried out to answer specific questions, and sample surveys answer questions about the whole population. Researchers are not interested in the sample itself, but in what they can learn from it than can be applied to the whole population.
A sample survey will usually offer greater scope than a census. This may mean, for example, that it's possible to study the population of a larger (geographical) area, or to find out more about the same population by asking a greater variety of questions, or to study the same area in greater depth.
Development of a sample survey
Of course, whatever the potential advantages of sample surveys, they will not be realised unless the sample survey is correctly defined and organised. If we ask the wrong 'people' the wrong 'questions' we will not get a useful estimate of the characteristics of the population. Sample surveys have advantages provided they are properly designed and conducted. The first component is to plan the survey (and select the...
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