SPSS Survival Manual: Version 12

SPSS Survival Manual: Version 12

SPSS Survival Manual: Version 12

SPSS Survival Manual: Version 12

Synopsis

A new edition of the internationally successful, user-friendly guide that takes students and researchers through the often daunting process of analysing research data with the widely-used SPSS software package. Fully revised and updated for SPSS Version 12, it features examples from a wide range of disciplines.

Excerpt

When you are selecting scales to include in your study it is important to find scales that are reliable. There are a number of different aspects to reliability (see discussion of this in Chapter 1). One of the main issues concerns the scale's internal consistency. This refers to the degree to which the items that make up the scale 'hang together'. Are they all measuring the same underlying construct? One of the most commonly used indicators of internal consistency is Cronbach's alpha coefficient. Ideally, the Cronbach alpha coefficient of a scale should be above .7. Cronbach alpha values are, however, quite sensitive to the number of items in the scale. With short scales (e.g. scales with fewer than ten items), it is common to find quite low Cronbach values (e.g. .5). in this case it may be more appropriate to report the mean interitem correlation for the items. Briggs and Cheek (1986) recommend an optimal range for the inter-item correlation of .2 to .4.

The reliability of a scale can vary depending on the sample that it is used with. It is therefore necessary to check that each of your scales is reliable with your particular sample. This information is usually reported in the Method section of your research paper or thesis. If your scale contains some items that are negatively worded (common in psychological measures), these need to be 'reversed' before checking reliability. Instructions for how to do this are provided in Chapter 8. Before proceeding, make sure that you check with the scale's manual (or the journal article that it is reported in) for instructions concerning the need to reverse items and also for information on any subscales. Sometimes scales contain a number of subscales that may, or may not, be combined to form a total scale score. If necessary, the reliability of each of the subscales and the total scale will need to be calculated.

Details of example

To demonstrate this technique I will be using the survey.sav data file included on the website accompanying this book (see p. xi). Full details of the study, the questionnaire and scales used are provided in the Appendix. If you wish to follow along with the steps described in this chapter you should start spss and open the file labelled survey.sav. This file can be opened only in spss. in the procedure described below I will explore the internal consistency of one of the scales from the questionnaire. This is the Satisfaction with Life scale, which is made up of five items. These items are labelled in the data file as follows: lifsat 1, lifsat 2, lifsat 3, lifsat 4, lifsat 5.

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