Scales of Measurement

continuous variable

a quantitative variable that has an infinite number of possible values that are not countable

Discrete variables

assume values that can be counted

Error Variable

Anything besides the thing measured by the test. Extra info that cannot be quantified in the context of the experiment.

nominal variables

variables measured in monetary units

ordinal variable

a qualitative variable that incorporates an ordered position, or ranking (HS class rank, sports ranks..where you fall among the others)

interval variable

data measured on a scale along the whole of which intervals are equal (score on a personality or IQ test); Has no true zero

Many Psychological constructs are actually _______, but they approximate interval enough that we treat them that way.



Optimal for the most number of analyses; has a true zero point; "non" can occur and has meaning. (performance on a timed test [you can score 0]; neurological funtioning)

measures of central tendency

mean, median, mode

measures of variability

range, variance, standard deviation

interquartile range

The difference between the upper and lower quartiles.


(n.) - a difference between what is expected and what actually occurs

1 standard deviation

68% of data

2 standard deviations

95% of data

3 standard deviations

99.7% of data


A set of scores


A graph of vertical bars representing the frequency distribution of a set of data.

bimodal distribution

a frequency distribution having two different values that are heavily populated with cases (Crossing a bridge during rush hour in the morning and evening)

positively skewed distribution

A distribution in which scores pile up at the low end of the scale (If a test is too hard, few people will do well)

negatively skewed distribution

A distribution in which most scores pile up at the high end of the scale (A test too easy)

normal curve

the bell-shaped curve that describes the distribution of many physical and psychological attributes. Most scores fall near the average, and fewer and fewer scores lie near the extremes


Uses standards based on the performance of certain groups.

Performance Criteria

sets a certain "bar" to see if it has been met (Cut score)

test age equivalent

Equate the test-takers performance with the average performance of the same age group.

grade equivalent score

Test score indicating the grade level of students to whom a test taker performed most similarly.


Measures of central tendency that divide a group of data into 100 parts.

normalizing a distribution

involves "stretching" the skewed curve into the shape of a normal curve and creating a corresponding scale of standard scores


Standard score with a mean of 5 and a standard deviation of 2; it is always reported as a whole number.

Z score

a type of standard score that tells us how many standard deviation units a given score is above or below the mean for that group

T score

standard score with a mean of 50 and a standard deviation of 10 (MMPI-2-RF uses T scores)