Psychological Measurement

1
Psychological Measurement

• Dr. John T. Drea
• Professor of Marketing
• Western Illinois University

2
Basic Measurement Concepts

• Always remember that what you collect is a
  measurement score - it is not the true level of a
  construct.
• True score (construct) actual score
  (measure) error
• Error refers to any type of variation in the
  actual score that is not attributable to
  variation in the true score.
• The best measurement technique is the one which
  satisfies the research question while closely
  approximating the true score.

3
Basic Measurement Concepts

• Outside sources of variation are commonly
  referred to as bias.
• To reduce bias, remember
• Clearly define what it is you want to measure
  (e.g., theres a difference in attitude toward a
  brand and satisfaction in using the brand)

4
Basic Measurement Concepts (cont.)

• Reducing bias
• Research subjects must have the ability to answer
  the stated question.
• Research subjects must perceive the data
  collection instrument as intended.
• Research subjects must be motivated to provide
  the an actual score in such a way it parallels
  their true score.
• The response categories provided must closely
  approximate the true score of the individual.

5
Should You Use Single or Multi-Item Measures?

• It depends on the complexity of what is being
  measured.

Multi-item Measures
Single-item Measure
Actual-1
Actual-2
TRUE
TRUE
Actual
Actual-3
Think of trying to represent an object by
photographing it from one angle vs. several
different angles.
6
Reliability

• Refers to the similarity of results provided by
  independent but comparable measures of the same
  object, trait, or construct.
• Reliability is based on the use of maximally
  similar methods.
• If we measure X using scale Y and we do this
  three times, will we get effectively the same
  result every time?
• Can be assessed by measuring an individual at two
  points in time.
• Most common means of assessing reliability is
  through Cronbachs alpha.

7
Reliability (continued)

• Cronbachs (coefficient) alpha
• A measure of the internal homogeneity of a set of
  scale items.
• Assesses the degree to which a group of scale
  items correlate with a representation of the
  score.
• Acceptable levels are 0.70 for exploratory
  research, 0.80 for applied research (Nunnally)
• Look at item-to-total correlations to improve
  levels of reliability.

8
Validity

• Refers to the extent to which differences in
  scores on (a measure) reflect true differences
  among individuals on the characteristic we seek
  to measure, rather than constant or random
  errors. (Selltiz, Wrightsman, and Cook 1976)
• The burden is on the researcher to establish that
  a measure captures the characteristic of
  interest.
• The relationship between a measured score and a
  true score is inferred but never established
  unequivocally. The basis for this inference
• Direct assessment of validity
• Indirect assessment of reliability

9
Types of Validity

• Pragmatic Validity How well does the measure
  predict?
• Content (Face) Validity Do the construct and the
  measure match up in a superficial manner?
• Construct Validity What is the instrument
  actually measuring?
• Convergent Validity two different measures of
  the same construct are highly correlated with one
  another
• Discriminant validity two measures of different
  constructs do not correlate highly
• Nomological validity the measure behaves in a
  way consistent with theory/prior experience
• The assessment of validity should involve the use
  of maximally different methods.