Chapter 14 Coefficient Alpha
Coefficient Alpha [also known as Cronbach’s Alpha, but Cronbach disliked that association; Cronbach and Shavelson (2004)] is considered a measure of internal consistency and can, if a set of severe assumptions is met, estimate the reliability of a set of items.
14.1.1 Example dataset
This example uses the Rosetta Stats example dataset “pp15” (see Chapter 1 for information about the datasets and Chapter 3 for an explanation of how to load datasets).
14.2 Input: jamovi
In the “Analyses” tab, click the “Factor” button and from the menu that appears, select “Reliability Analysis” as shown in Figure 14.1.
In the box at the left, select all variables you want to include in this analysis and move them to the box labelled “Items” using the button labelled with the rightward-pointing arrow as shown in Figure 14.2.
Because Coefficient Alpha (called Cronbach’s Alpha in the jamovi interface) has already been checked (see the left-most column at the bottom labelled “Scale Statistics”), you will immediately see Coefficient Alpha for the selected items appear in the table in jamovi’s Results pane on the right-hand side. You can now also order additional statistics, such as the value Coefficient Alpha would have if you were to omit each item, as shown in Figure 14.3.
14.3 Input: R
14.4 Input: SPSS
To compute Coefficient Alpha in SPSS, use the following command:
RELIABILITY /VARIABLES = highDose_AttGeneral_good highDose_AttGeneral_prettig highDose_AttGeneral_slim highDose_AttGeneral_gezond highDose_AttGeneral_spannend /MODEL = ALPHA.
To order additional information, such as descriptive statistics, inter-item correlations, and other scale statistics, you can specify additional options:
RELIABILITY /VARIABLES = highDose_AttGeneral_good highDose_AttGeneral_prettig highDose_AttGeneral_slim highDose_AttGeneral_gezond highDose_AttGeneral_spannend /MODEL = ALPHA /STATISTICS = DESCRIPTIVE SCALE CORR /SUMMARY = TOTAL.
14.6 Output: R
14.6.1 R: rosetta
188.8.131.52 Reliability analysis
184.108.40.206.1 Scale structure
220.127.116.11.1.1 Scale structure
18.104.22.168.1.1.1 Information about this scale
|Items:||highDose_AttGeneral_good, highDose_AttGeneral_prettig, highDose_AttGeneral_slim, highDose_AttGeneral_gezond & highDose_AttGeneral_spannend|
|Number of correlations:||10|
|Percentage positive correlations:||100|
22.214.171.124.1.1.2 Estimates assuming interval level
|Revelle’s Omega (total):||0.79|
|Greatest Lower Bound (GLB):||0.87|
Note: the normal point estimate and confidence interval for omega are based on the procedure suggested by Dunn, Baguley & Brunsden (2013) using the MBESS function ci.reliability, whereas the psych package point estimate was suggested in Revelle & Zinbarg (2008). See the help (‘?ufs::scaleStructure’) for more information.