What Statistic Is Used to Compute Internal Reliability

Cronbach'southward Blastoff (α) using SPSS Statistics

Introduction

Cronbach's blastoff is the most common measure of internal consistency ("reliability"). Information technology is most commonly used when y'all have multiple Likert questions in a survey/questionnaire that class a scale and you wish to determine if the scale is reliable. If y'all are concerned with inter-rater reliability, we also have a guide on using Cohen's (κ) kappa that you might observe useful.

SPSS Statistics

Instance

A researcher has devised a nine-question questionnaire to measure out how safety people feel at work at an industrial circuitous. Each question was a 5-point Likert item from "strongly disagree" to "strongly agree". In order to understand whether the questions in this questionnaire all reliably measure the same latent variable (feeling of safe) (so a Likert calibration could be synthetic), a Cronbach's blastoff was run on a sample size of fifteen workers.

SPSS Statistics

Setup in SPSS Statistics

In SPSS Statistics, the nine questions have been labelled Qu1 through to Qu9. To know how to correctly enter your data into SPSS Statistics in society to run a Cronbach'due south alpha test, run across our guide: Entering Information into SPSS Statistics. Alternately, you tin can learn about our enhanced data setup content on our Features: Data Setup page.

SPSS Statistics

Test Process in SPSS Statistics

Cronbach's alpha can exist carried out in SPSS Statistics using the Reliability Assay... procedure. In this section, we set out this vii-footstep procedure depending on whether y'all take versions 26, 27 or 28 (or the subscription version of SPSS Statistics) or version 25 or an before version of SPSS Statistics. The latest versions of SPSS Statistics are version 28 and the subscription version. If you are unsure which version of SPSS Statistics y'all are using, see our guide: Identifying your version of SPSS Statistics. At the finish of these seven steps, nosotros show y'all how to translate the results from your Cronbach's blastoff assay.

SPSS Statistics versions 26, 27 and 28
(and the subscription version of SPSS Statistics)

1. Click Analyze > Scale > Reliability Analysis... on the top menu, equally shown below:

   Note: In version 27 and the subscription version, SPSS Statistics introduced a new look to their interface called "SPSS Lite", replacing the previous expect for versions 26 and before versions, which was chosen "SPSS Standard". Therefore, if you accept SPSS Statistics versions 27 or 28 (or the subscription version of SPSS Statistics), the images that follow will be light gray rather than bluish. Still, the procedure is identical in SPSS Statistics versions 26, 27 and 28 (and the subscription version of SPSS Statistics).

   

   Published with written permission from SPSS Statistics, IBM Corporation.

   
   You will be presented with the following Reliability Analysis dialogue box:
   

   

   Published with written permission from SPSS Statistics, IBM Corporation.

2. Transfer the variables Qu1 to Qu9 into the Items: box. Y'all can exercise this by drag-and-dropping the variables into their respective boxes or by using the button. Yous volition be presented with the following screen:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

3. Exit the Grandodel: option set equally , which instructs SPSS Statistics to run Cronbach'southward alpha.

   Note: If you want to provide a name for the scale, enter it in the Scale label: box. Since this only prints the proper name you enter at the top of the SPSS Statistics output, it is certainly non essential that yous do (in our example, we get out information technology blank).

4. Click on the push button. Y'all will be presented with the Reliability Analysis: Statistics dialogue box, as shown below:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

5. Select the Item, Scale and Scale if particular deleted options in the –Descriptives for– area, and the Correlations option in the –Inter-Item– area, as shown beneath:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

6. Click on the push. This will render you lot to the Reliability Analysis dialogue box.
7. Click on the button to generate the output.

At present that you take run the Cronbach's alpha process, we show yous how to interpret your results in the Interpreting Results section. Y'all can skip the section beneath, which shows you how to carry out Cronbach's blastoff when yous have SPSS Statistics version 25 or an earlier versions of SPSS Statistics.

SPSS Statistics version 25
and earlier versions of SPSS Statistics

1. Click Analyze > Scale > Reliability Analysis... on the top card, equally shown beneath:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

   
   You will exist presented with the following Reliability Analysis dialogue box:
   

   

   Published with written permission from SPSS Statistics, IBM Corporation.

2. Transfer the variables Qu1 to Qu9 into the Items: box. You tin do this by elevate-and-dropping the variables into their respective boxes or by using the push. You volition be presented with the following screen:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

3. Leave the Model: option set as , which instructs SPSS Statistics to run Cronbach'south alpha.

   Note: If you want to provide a name for the scale, enter it in the Scale label: box. Since this only prints the proper name you enter at the top of the SPSS Statistics output, it is certainly not essential that you lot exercise (in our example, nosotros exit information technology blank).

4. Click on the push button. You will exist presented with the Reliability Assay: Statistics dialogue box, as shown below:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

5. Select the Item, Scale and Scale if item deleted options in the –Descriptives for– surface area, and the Correfiftyations pick in the –Inter-Item– expanse, every bit shown below:

   

   Published with written permission from SPSS Statistics, IBM Corporation.

6. Click on the button. This will return you to the Reliability Analysis dialogue box.
7. Click on the button to generate the output.

Now that you have run the Cronbach's alpha procedure, we show you lot how to interpret your results in the next department.

SPSS Statistics

SPSS Statistics Output for Cronbach'south Blastoff

SPSS Statistics produces many unlike tables. The first of import table is the Reliability Statistics table that provides the actual value for Cronbach'southward alpha, as shown below:

Published with written permission from SPSS Statistics, IBM Corporation.

From our example, nosotros tin encounter that Cronbach's alpha is 0.805, which indicates a loftier level of internal consistency for our scale with this specific sample.

SPSS Statistics

Item-Total Statistics

The Item-Full Statistics tabular array presents the "Cronbach's Alpha if Item Deleted" in the terminal column, every bit shown below:

Published with written permission from SPSS Statistics, IBM Corporation.

This column presents the value that Cronbach's alpha would be if that particular item was deleted from the scale. We can see that removal of any question, except question 8, would upshot in a lower Cronbach's blastoff. Therefore, we would not want to remove these questions. Removal of question 8 would lead to a modest improvement in Cronbach's alpha, and we can also see that the "Corrected Item-Total Correlation" value was low (0.128) for this detail. This might lead usa to consider whether we should remove this item.

Cronbach's alpha simply provides y'all with an overall reliability coefficient for a set of variables (e.g., questions). If your questions reflect different underlying personal qualities (or other dimensions), for instance, employee motivation and employee commitment, Cronbach's alpha will not be able to distinguish between these. In order to do this and then check their reliability (using Cronbach'south blastoff), yous will first need to run a test such as a principal components analysis (PCA). You can acquire how to deport out principal components analysis (PCA) using SPSS Statistics, as well equally interpret and write up your results, in our enhanced content. You can acquire more than on our Features: Overview folio. Information technology is also possible to run Cronbach's alpha in Minitab.

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