GRAPPLING WITH DATA
Variability in observations
• Sources of variability
• measurement error and reliability
Visualizing the sample data
• Frequency distributions
– Plotting frequency distributions
– Types of distributions
FREQUENCY DISTRIBUTIONS
skewed
normal
bimodal
truncated
GRAPPLING WITH DATA
Reducing and describing the sample
• Measures of central tendency
– Mode: the most common score(s)
– Median: the middle score
– Mean: the “center of gravity” – sum / n
• Measures of dispersion
– Range: largest minus smallest score
– Variance: average squared deviation from mean
USING EXCEL FOR
DESCRIPTIVE STATISTICS
• Entering data into a spreadsheet
– Formatting numerical cells
• Generating frequency distributions
– Scores go in “data array”
– Categories go in “bins array”
– Select cells adjacent to bins + 1
– Select Frequency formula, data and bin arrays
– Press CTL+SHIFT+ENTER
USING EXCEL FOR
DESCRIPTIVE STATISTICS
• Obtaining measures of central tendency
– “by hand”
– By using Excel formulas
• Obtaining measures of variability
– Variance and standard deviation
TABLE ASSIGNMENT
• Obtain data file from web or lab
• Use Excel to calculate means, SD’s
• Construct table with those data for
– Three tasks (simple form, color, complex form)
– X four array sizes (4, 8, 16, 32)
• Format according to APA Style guide
Signal Detection Analysis
“no”
“yes”
Target absent
(noise)
Target present
(signal+noise)
Response
Criterion
SIGNAL DETECTION ANALYSIS
Response:
“YES”
“NO”
Stimulus Present
hits
misses
Stimulus Absent
false
alarms
correct
rejections
Deriving d’
d’
= NORMSINV(prob hit) - NORMSINV(prob FA)
In effect, the difference in “distance” of the mean of
the normalized distributions for hits versus false
alarms (!)
Signal Detection Analysis
Vary % of trials with targets:
10%
50%
90%
p(hit)
.24
.66
.94
p(FA)
.06
.36
.77
NormSI(h)
-0.71
0.41
1.55
NormSI(fa)
-1.55
-0.36
0.74
d’
0.85
0.77
0.82
Conclusion: likelihood of a “signal” affects response criterion (or
“bias”), not “sensitivity” to the signal.
FIGURE ASSIGNMENT
• Derive d’s from Hits & False alarm rates in
Lab 5
• Construct an APA-formatted Figure with:
– Mean d’ for 2 durations x 2 frame stimuli
– Error bars indicating standard deviations
– Figure caption on a separate page