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Chapter 7 - Analysing the Data Part IV - Analysis of Variance Chapter 1 - Behavioural Science and research Chapter 2 - Research Design Chapter 3 - Collecting the Data Chapter 4 - Analysing the Data Part I - Descriptive Statistics Chapter 5 - Analysing the Data Part II - Inferential Statistics Chapter 6 - Analysing the Data Part III - Common Statistical Tests Sources of data Samples and populations Sampling methods Quantitative and qualitative approaches Questionnaire design Coding of data Readings and links


Chapter 3: Collecting the Data


Samples and populations

The following is an adapted extract from Tabachnick and Fidell (1989):

Samples are measured in order to make generalisations about populations. Ideally, samples are selected, usually by some random process, so that they represent the population of interest. In real life, however, populations are frequently best defined in terms of samples, rather than vice versa; the population is the group from which you were able to randomly sample.

Sampling has somewhat different connotations in non-experimental and experimental research. In non-experimental research, you investigate relationships among variables in some predefined population. Typically you take elaborate precautions to ensure that you have achieved a representative sample of that population; you define your population, then do your best to randomly sample from it.

In experimental research, you attempt to create different populations by treating subgroups from an originally homogeneous group differently. The sampling objective here is to assure that all subjects come from the same population before you treat them differently. Random sampling consists of randomly assigning subjects to treatment groups (levels of the IV) to ensure that, before differential treatment, all samples come from the same population. Statistical tests provide evidence as to whether, after treatment, all samples still come from the same population. Generalisations about treatment effectiveness are made to the type of subjects who participated in the experiment.




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