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