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Chapter 2

Psychological Assessment - Chapter 2 Book Notes

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Norms and Basic Statistics for Testing
Using number systems allow us to manipulate information
Why We Need Statistics?
Statistical methods serve two important purposes to aid in scientific understanding
Statistics are used to make descriptions
Numbers provide convenient summaries and allow us evaluate some observations relative to
Statistics are used to make inferences: which are logical deductions about events that cannot be
observed directly
First, clues are gathered and displayed (also called exploratory data analysis)
Then, clues are evaluated against rigid statistical rules, through a process called confirmatory
data analysis
Descriptive statistics: are methods used to provide a short description of a collection of quantitative
Inferential statistics: are methods used to make inferences from observations of a small group of people
known as a sample to a larger group called a population
Scales of Measurement
Measurement: the application of rules for assigning numbers to objects
Example - wine may be rated on a 10-point scale where 1 means extremely bad and 10 means extremely
Properties of Scales

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Magnitude, equal intervals, and an absolute 0 make scales different from one another
Magnitude: is the property of moreness
A scale has the property of magnitude if a particular instance of the attribute represents more,
less, or equal amounts of the given quantity than does another instance
When a coach assigns identification numbers to teams, it does not have this property
When a coach ranks the team by the number of games they have won, the property of magnitude
Equal intervals: exists if the difference between two points at any place on the scale has the same meaning
as the difference between two other points that differ by the same number of scale units
Psychological tests rarely have equal intervals
The difference between an IQ of 65 and 70 is not the same as the difference between 85 and 90
When a scale has the property of equal intervals, the relationship between the measured units and
some outcome can be expressed by a straight line or a liner equation in the form of y= mx + b
Absolute 0: is obtained when nothing of the property being measured exists
When heart rate is 0, the individual has died
Extremely difficult for many psychological qualities to have an absolute 0 point
Types of Scales
Nominal: the lowest measurement scale
Placement of data into categories, without any order or structure

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Purpose is to name objects and used when the information is qualitative rather than quantitative
No magnitude, no equal intervals, no absolute 0
Ordinal: allows you to rank individuals or objects but no meaning of the differences between the ranks
Ranking five types of beer from most flavorful to least flavorful
No equal intervals, no absolute 0
For most problems in psychology, ordinal scales are used
Interval: has a magnitude and equal intervals, but no absolute zero
Temperature has the property of magnitude because 60C is warmer than 45C
The difference between 100C and 90C is equal to a similar difference of 10C at any point on the
An absolute zero does not exist, because zero on the scale represents the freezing point of water
Ratio: has a magnitude, equal intervals, and an absolute zero
Allows ratios of numbers to be meaningfully interpreted, such as the ratio of Johns height to
Sarahs height is 1.52
The speed of travel has an absolute zero, and if you are driving onto a highway you speed
increases by double the amount
Levels of measurements defines which mathematical operations we can apply to numerical data
Nominal data - each observation can placed in only one mutually exclusive category (example,
you are a member of only one gender)
Used to create frequency distributions
Ordinal data - can be manipulated, but the result is difficult to interpret because it does not reflect
the magnitude of the manipulated observations nor the true amounts of the property
If the weights of 10 children are rank ordered, knowing the rank does not reveal how tall he or she
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