# What is Spearman Brown reliability index?

**Spearman**-

**Brown**Formula (also called the

**Spearman**-

**Brown**Prophecy Formula) is a measure of test

**reliability**. It's usually used when the length of a test is changed and you want to see if

**reliability**has increased. The formula is: r

_{kk}= k(r

_{11}) / [1 + (k – 1)* r

_{11}]

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Also question is, what does the Spearman Brown formula do?

The **Spearman**–**Brown** prediction **formula**, also known as the **Spearman**–**Brown** prophecy **formula**, **is** a **formula** relating psychometric reliability to test length and used by psychometricians to predict the reliability of a test after changing the test length.

Furthermore, what is considered a good reliability coefficient? The closer each respondent's scores are on T1 and T2, the more reliable the test measure (and the higher the **coefficient** of stability will be). Between 0.9 and 0.8: **good reliability**. Between 0.8 and 0.7: acceptable **reliability**. Between 0.7 and 0.6: questionable **reliability**.

Keeping this in consideration, what is reliability index?

**Item reliability** is simply the product of the standard deviation of **item** scores and a correlational discrimination **index** (**Item**-Total Correlation Discrimination in the **Item** Analysis Report). So **item reliability** reflects how much the **item** is contributing to total score variance.

How do you determine reliability?

Correlate the test scores of the two administrations of the same test. – Parallel Forms **Reliability**: Determines how comparable are two different versions of the same measure. To calculate: Administer the two tests to the same participants within a short period of time. Correlate the test scores of the two tests.