Intro to Assessment - Standardization, Norms, and Score Interpretation

Variability

Psychological and cognitive measures

• Many are designed to measure differences between people.

• Thus, they must have variability (i.e., differences) in the score

• Variance and standard deviation measure variability by indicating

  the degree to which scores (overall) differ from the mean

Standard Deviation

A measure of how much scores vary, or deviate, from the mean.

Normal Curve

A frequency distribution that, when graphed, forms a bell-shaped curve

• Most scores are the mean

• The further away from the mean a score is, the rarer it becomes. 

• Symmetrical distribution of scores with an equal number of scores above (to the right of) and below (to the left of) the midpoint. 

The Three-Sigma Rule

68-95-99.7 rule

• 68.27% of the values lie within one standard deviation of the mean

• 95.45% of the values lie within two standard deviations of the mean

• 99.7% of the values lie within three standard deviations of the mean

Types of Scores

• Raw score

• Standard scores (e.g., scaled scores, z-scores, t-scores)

• Percentile ranks

Derived Scores 

An individual's standing relative to the norm group.

• All derived scores are obtained from raw scores.

• Derived scores are different expressions of an individual’s performance

  • t-scores

  • z-scores

  • standard scores

z-Scores

• Used in assessment measures

• Mean = 0

• SD = 1

t-Scores

• Used in personality and behavioral measures

• M = 50

• SD = 10

Standard Scores

Raw scores that have been transformed so that they have a predetermined mean and standard deviation

• Used in cognitive measures

• Index: M = 100, SD = 15 (WISC Composite Score)

• Subtest: M = 10, SD = 3 (WISC Scaled Score)

• Most scores will fall between 55 and 145.

  • Anything above or below that is special.

Percentile Ranks 

Derived scores that permit us to determine an individual’s position relative to the standardization sample

Interpreting Scores

• Raw score

• Scaled score

  • Raw score into standardized score

• Composite score

  • Used to determine how well the individual performed compared to the norm group

• Percentile ranking

• Confidence interval

  • The percentage (usually 95) of chance of a range of scores