MEASUREMENT
• An attempt to provide an objective
estimate of a natural phenomenon
– e.g. measuring height
– or weight
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Measurement scales
• Nominal Scales
– Categories -- mutually exclusive and collectively exclusive.
• Ordinal Scales
– Rank order – e.g. most important to least important.
• Interval Scales
– Ordinal + the intervals between data points are equal.
• Ratio Scales
– Above + a meaningful absolute zero or origin.
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Measurement scales: Example
• How could you measure height using
the following levels of measurement?
– Nominal Scales
– Ordinal Scales
– Interval Scales
– Ratio Scales
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Measurement error
Measuring my weight with a scale
1.
I stand on my bathroom scale
It reads “160”
2.
I stand on it again
It reads “159”
3.
I stand on it again
It reads “160”
What’s the best estimate of my weight?
How reliable is my scale?
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Reliability
• Consistent results over time.
• Measures are free from random error.
Test-Retest Reliability:
Obtained by repeating the measurement
using the same instrument under as
nearly the same conditions as possible.
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Validity
Addresses the issue of whether what we
tried to measure was actually measured.
How do I know if my bathroom scale is
valid?
That is, does 160 really MEAN anything?
How could I know this with my bathroom scale?
1. JUST using the same scale
2. Using ANOTHER scale.
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Types of Validity
• Face validity
• Content validity
• Criterion-Related Validity
• Predictive Validity
• Concurrent Validity
• Construct Validity
• Convergent Validity
• Discriminant Validity
All types of validity are related.
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Reliability and Validity Illustrated
• Situation 1:
• A lack of reliability
• my scale varies a lot (150, 160, 170)
• Situation 2:
• High reliability, but lacks validity
• My scale always shows 160, but other scales are
different
• Situation 3:
• Reliable and valid
• ALL scales show a consistent 160
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Scaling
Scaling Defined
In order to decrease noise:
you measure something several times
• Unidimensional scaling
• Multidimensional scaling
• Graphic Rating Scales
Present to respondents with a graphic continuum
typically anchored by two extremes
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Types of scales
Rank Order Scale
Judging one item against another
Q-Sorting
Sort into piles according to categories
Paired Comparisons
Picking one of two objects
The Semantic Differential
The researcher selects opposite pairs
Likert Scales
Express either a favorable or unfavorable attitude
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The End
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