Psychometrics in Neuropsychological Assessment (Strauss, 2006) Flashcards
How do you calculate a z-score?
obtained score - sample mean/ sample SD
What is the mean and SD of a Z-score?
Mean = 0 SD = 1
What is a Z-score?
A type of standard score
What does a Z-score quantify?
How many SDs a score is from the mean
What is a t-score?
Another linear transformation of a raw score
What is the mean and SD of a t-score?
Mean = 50 SD = 10
Why do we use standard scores?
By virtue of conversion to a common metric, they facilitate the comparison of scores across measures
What must the distribution of the tests be for us to use standardized scores?
Approximately normal
What are two things that must be considered before comparing test scores?
- The reliability of the 2 measures
2. Their intercorrelation
How can you calculate the prevalence value from a z-score?
- Look up the corresponding estimated frequency (e.g., -4) in a z-score table
- Divide 1 by that value (e.g., .00003/1 = 31,560)
Thus, the estimated prevalence of -4 is 1 in 31,560
A test with a normal distribution in the general population may show extreme skew or other divergence from normality when administered to a population that differs considerably from the average individual. Give an example.
Vocab test being negatively skewed when administered to doctoral students in literature vs positively skewed when given to preschoolars who recently immigrated
When a new test is constructed, how can non-normality be corrected?
By examining the distribution of scores on the prototype test, adjusting test properties and resampling until a normal distribution is reached
What is another way of saying negatively skewed (in terms of testing)?
low ceiling
What is another way of saying positively skewed (in terms of testing)?
high ceiling
Will a large N correct for non-normality of an underlying population distribution?
No - a larger sample will only produce a more normal distribution if the underlying population distribution from which the sample was obtained is normal
What factors may lead to non-normal test score distributions?
- The existence of discrete subpopulations within the general population with differing abilities
- Ceiling or floor effects
- Treatment effects that change the location of means, medians and modes and affect variability and distribution shape
Small samples may yield non-normal distributions due to what?
Random sampling effects
What is a formal measure of asymmetry?
Skewness
What is the skew value of a true normal distribution?
0
What will have a skew value NEAR 0?
A non-normal but symmetric distribution
What do negative skew values indicate?
That the left tail of the distribution is heavier than the right
What does skewness tell us about the mean and median?
If there is skewness then the mean and median are not identical because the mean will not be at midpoint in rank
Z-scores will not accurately translate into sample percentile rank values
What increases as skew increases?
Error in mapping z-scores to sample percentile ranks
What kind of distributions often have significant skew? Give an example.
Truncated distributions often have significant skew
Truncated distributions often occur when range is restricted: e.g., reaction time
Floor and ceiling effects may be defined as what?
The presence of truncated tails in the context of limitations in range of item difficulty
What does a high floor mean?
All items difficult
What does a low ceiling mean?
All items easy
What does multimodality refer to?
The presence of more than one peak in a frequency distribution
Non-normality has major implications for interpreting and comparing standard scores - elaborate.
Standardized scores derived by linear transformation will not correspond to sample percentiles and the degree of divergence can be quite high
Normalizing transformations introduce error - when is it acceptable to normalize scores?
1) If they come from a large and representative sample
2) If any deviation from normality arises from defects in the test rather than characteristics of the sample
Normalizing transformations introduce error, what is the preferable way to handle non-normality in scores?
Preferable to adjust score distributions prior to normalization by modifying test content rather than statistically transforming non-normal scores into a normal distribution
Define reliability
The consistency of measurement of a given test and can be defined in several ways, including consistency within itself (internal consistency reliability) and consistency over time (test-retest reliability)
What do indices of reliability show?
The degree to which a test is free from measurement error
Internal consistency reliability
The tests reliability with itself
Reliability coefficients are influenced by what two things?
- Test characteristics
Sample characteristics
Reliability coefficients are influenced by test characteristics - list some examples of test characteristics
Length, item type, item homogeneity, influence of guessing
Reliability coefficients are influenced by sample characteristics - list some examples of sample characteristics
Sample size, range, variability
Test clarity is closely related to reliability. What is meant by test clarity?
- Clearly written
- Easily understood instructions
- Standardized administration conditions
- Exploring scoring rules that minimize subjectivity
A process for training raters