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en:general:indices [02.04.2015 17:32] – [Frequently Asked Questions] oezbf2012 | en:general:indices [02.04.2015 17:33] – oezbf2012 | ||
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**Can I always combine variables to an index?** | **Can I always combine variables to an index?** | ||
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The calculation of a scale index is only sensible if all items reflect the same construct. In practice you can test this by computing the correlation between the items by means of Cronbach' | The calculation of a scale index is only sensible if all items reflect the same construct. In practice you can test this by computing the correlation between the items by means of Cronbach' | ||
As a rule of thumb Cronbach' | As a rule of thumb Cronbach' | ||
**Do I have to z-standardize the items before calculating the scale index?** | **Do I have to z-standardize the items before calculating the scale index?** | ||
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It depends on the scale. The z-standardization has one drawback: The range of the scale index is not the same as for the individual items. This complicates the interpretation: | It depends on the scale. The z-standardization has one drawback: The range of the scale index is not the same as for the individual items. This complicates the interpretation: | ||
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**Does the correlation of a construct with other constructs depend on the number of items?** | **Does the correlation of a construct with other constructs depend on the number of items?** | ||
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Yes and no. In principle the size of the correlation is independent from the number of items. However, in a neatly constructed scale the quality of the measurement is increased by the number of items. Therefore the scale index contains fewer measurement errors and as a result higher correlations may be observed. | Yes and no. In principle the size of the correlation is independent from the number of items. However, in a neatly constructed scale the quality of the measurement is increased by the number of items. Therefore the scale index contains fewer measurement errors and as a result higher correlations may be observed. | ||
To the contrary a correlation based on more items can actually be overrated if both constructs underlie the same measurement error. The higher correlation with more items is a spurious correlation in this case – e.g. because some people prefer to answer on the right end of the scale (acquiescence). | To the contrary a correlation based on more items can actually be overrated if both constructs underlie the same measurement error. The higher correlation with more items is a spurious correlation in this case – e.g. because some people prefer to answer on the right end of the scale (acquiescence). | ||
**What is the measurement level of scale indices?** | **What is the measurement level of scale indices?** | ||
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Interval scales (metric). For the computation of a mean or a sum you have to assume that you items are at least approximately interval scaled (quasi-metric). Consequently, | Interval scales (metric). For the computation of a mean or a sum you have to assume that you items are at least approximately interval scaled (quasi-metric). Consequently, | ||
**Are scales with reversed-polarity items preferable? | **Are scales with reversed-polarity items preferable? | ||
- | This question cannot be answered in general. | + | |
+ | There is no general | ||
The use of reversed-polarity items will usually result in the effect that the correlation between items (Cronbach' | The use of reversed-polarity items will usually result in the effect that the correlation between items (Cronbach' |