As an expert in the field of psychometrics and assessment, I can provide a detailed explanation regarding the relationship between index scores and standard scores. It's important to understand the nuances and differences between these two types of scores, as they are used in various contexts within psychological and educational assessments. Index Scores vs. Standard Scores An index score is a composite measure derived from combining two or more subtest scores. These subtests are typically related and are combined to provide a more comprehensive understanding of a particular cognitive or psychological construct. For example, in an intelligence test, an index score might combine verbal and non-verbal reasoning subtests to measure a broader concept like general cognitive ability. On the other hand, a standard score is a type of scaled score that has been adjusted in a way that the distribution of scores has a specific mean and standard deviation. The most common standard score system has a mean of 100 and a standard deviation of 15. This is known as a z-score normalization, where the mean is set to 100 and the standard deviation to 15, allowing for easy interpretation of how an individual's score compares to the general population. Key Differences 1. Composition: Index scores are composites of multiple subtests, while standard scores are single measures that have been scaled. 2. Mean and Standard Deviation: Index scores, like composite, cluster, and some other types of scores, often have a mean of 100 and a standard deviation of 15, similar to standard scores. However, this does not mean they are the same. The similarity in statistical properties is by design to facilitate comparison but the context and purpose of each score type differ. 3. Purpose: Index scores are used to provide a broader view of a person's abilities within a specific domain, while standard scores are used to compare an individual's performance to a reference group. 4. Interpretation: Standard scores are straightforward to interpret because they are based on the individual's performance on a single test. Index scores require a more nuanced interpretation because they represent a combination of abilities. 5. Reliance: It's cautioned not to rely solely on composite scores like index scores to measure a child's skills. This is because while they provide a snapshot, they may not capture the full range of a child's abilities, especially if there are significant variations within the subtests that comprise the index. Conclusion While index scores and standard scores share some statistical properties, they are not the same and serve different purposes in assessment. Index scores offer a synthesized view of abilities across multiple areas, and standard scores provide a direct comparison of an individual's performance to a normative group. It's crucial for practitioners to understand these differences to accurately interpret and apply assessment results. read more >>

Two or more subtests are often combined into a single score called a composite score, a cluster score, or an index score. Most composite, cluster, and index scores have a mean of 100 and a standard deviation of 15. Do not rely solely on composite scores to measure your child's skills.read more >>