Lecture6 - University of Idaho
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Transcript Lecture6 - University of Idaho
PSYC512: Research Methods
Lecture 6
Brian P. Dyre
University of Idaho
PSYC512: Research Methods
Lecture 5 Outline
Questions about material covered in Lecture 5
Scientific Method: Proof and disproof & Strong
Inference
Operational definitions
Issues in Measurement
Choosing Measures
Scales of Measurement
Variables: Reliability and validity
sampling
PSYC512: Research Methods
Choosing Measures
Research tradition
e.g., operant conditioning—lever pressing
e.g., cognition—accuracy and reaction time
e.g., sensation and perception—discrimination
accuracy
e.g., personality—surveys, inventories (self-reports)
Theory
e.g., the psychophysical postulate – discrimination
accuracy
e.g., Serial vs. parallel processes in visual search – RT
Availability of new techniques
Availability of equipment
PSYC512: Research Methods
Features of Measures: Scale of
Measurement (Stevens, 1946)
Four types: nominal, ordinal, interval, and ratio
Nominal scales
set of unique cases, types, or categories with NO ORDER
Only non-parametric operations are valid: counting frequencies,
modes, chi-square, point-biserial correlation
Ordinal scales
different categories that can be ranked along a continuum
more or less, but not how much more or less
Only non-parametric operations are valid : counting
frequencies, modes, medians, chi-square, rank-order
correlation
PSYC512: Research Methods
Features of Measures: Scale of
Measurement (Stevens, 1946)
Interval
intervals of the scale are equal in magnitude
Necessary but not sufficient condition for parametric statistical
tests
valid operations: all mathematical operations, means, standard
deviations, etc. may be calculated
If other distributional assumptions are met: linear and non-linear
regression, t-tests, ANOVA are also valid
no fundamental zero—no ratio statements allowed
Ratio
Like interval but also has a fundamental zero point—allows ratio
statements such as “A is twice as much as B”
Generally interval or ratio scales should be used if possible
More powerful and flexible statistical tests
More precision in evaluating quantitative hypotheses
PSYC512: Research Methods
Features of Measures:
Sensitivity
Sensitivity: measure must show changes in response
to changes in the independent variable
Range effects
Ceiling effects: variable reaches its highest
possible value and gets truncated (test is too
easy)
Floor effects: variable reaches its lowest possible
value and gets truncated (test is too hard)
PSYC512: Research Methods
Features of Measures:
Reliability
the ability of a measure to produce consistent results when
repeated measurements are taken under identical conditions
Types:
precision: physical measurement (1/noise)
margin of error: sampling in surveys
interrater reliability: observers viewing the same behavior
Test-retest, parallel forms and split-half reliabilities:
psychological tests
PSYC512: Research Methods
Other Features of Measures
Accuracy
does a measure produce results that agree with a
known standard?
Accuracy vs. Precision
Validity
Measurement validity: the extent to which your
measure indeed measures what it is intended to
measure
Types: Face validity, Content validity, Criterionrelated validity (concurrent vs. predictive),
Construct validity
Relationship between reliability and validity
PSYC512: Research Methods
Probability and Statistics
Why are probability and statistics important?
Used to assess variability in data
Treatment Variance
Variability due to different levels of independent
variable
Good variance that we want to maximize
Error Variance
Variability in data due to factors other than the
treatment
Bad variance that we want to minimize
Probability and Statistics are simply tools used to assess and
compare these sources of variability
PSYC512: Research Methods
Visualizing Variability: Distributions
of Frequency and the Histogram
Histograms: used to represent
frequencies of data in different
classes or categories
Bin
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Frequency
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PSYC512: Research Methods
Frequency
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Displaying Histograms:
Stem and Leaf Plots
Stem and Leaf plots are used to display histograms
graphically (on their side) using only typed characters
Stem
Leaf
(hypothetical histogram for IQ)
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012234445555667777889
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PSYC512: Research Methods
Distributions of Probability Density
Similar to frequency
histogram except y-axis
now represents
probability density
(mass) rather than
frequency
Probability density =
Frequency/N
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Probability Density
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Some Types of Distributions
Gamma
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Probability Density
Probability Density
Normal
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PSYC512: Research Methods
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Measures of the Center of a
Distribution
Measures of center represent the general
magnitude of scores in a distribution
Mode: most frequent score
Median: the middle score of an ordered
distribution
Mean (average):
where X is the data and
N is the total number of
observations
X
N
PSYC512: Research Methods
Measures of the Spread of a
Distribution
Measures of spread are used to assess the
consistency of scores in a distribution
Range = max score – min score
Interquartile range = score(Q3) – score(Q1)
Variance (s2) and standard deviation (s)
where X is the data,
2
m is the mean of the data,
X
and N is the total number
s2
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of observations
PSYC512: Research Methods
More on Variance
Standard Deviation (s) = sqrt(variance)
where X is the data,
2
X
m is the mean of the data,
s
and N is the total number
N
of observations
Why N instead of N-1? Populations vs. Samples
Remembering how to compute variance
“the mean of the squares – square of the means”
s
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PSYC512: Research Methods
Describing Distributions
Parametrically: Statistical Moments
Any distribution based on interval or ratio data can be
summarized by its statistical moments
First Moment: Mean—location of distribution on x-axis
Second Moment: Variance—dispersion of distribution
Third Moment: Skewness—symmetry of distribution
Fourth Moment: Kurtosis—degree of “peakedness”
PSYC512: Research Methods
Estimators
Sample statistics estimate population parameters
Mean: M or X vs. M
Variance: s2
vs. s2
Properties of Estimators
Sufficiency: uses all information in sample (mean and variance
are sufficient, mode and range are not)
Unbiasedness: expected value approaches real value with
increased sampling
Efficiency: tightness of cluster of sample statistics relative to
the population parameter
Resistance: influence of outliers on sample statistic
PSYC512: Research Methods
Next Time…
Topic: descriptive statistics, variables, sampling,
and more on hypothesis testing
Be sure to:
Read the assigned readings (Howell chapters
3-4)
Continue searching and reading the scientific
literature for your proposal
PSYC512: Research Methods