PS 366
4

Measurement
• Related to reliability, validity:
• Bias and error
– Is something wrong with the instrument?
– Is something up with the thing being measured?

Measurement
• Bias & error with the instrument
– Random?
– Systematic?

Measurement
• Bias & error with the thing being measured
– Random?
• failure to understand a survey question
– Systematic?
• does person have something to hide?

Measurement
• Example:
– Reliability, validity, error & bias in measuring unemployment
– Census survey [also hiring reports, claims filed w/ government, state data to feds...]
– What sources of bias?

Measurement
• Unemployment [employment status]:
– Fully employed
– Part time
– looking for work, + part time
– looking for work, no job
– lost job, not looking for work
– retired

Measurement
• Example:
– Reliability, validity, error & bias in measuring victims of violent crime
– Census surveys, police records, FBI UCR
– What sources of bias?

Measurement
• How do we ask people questions about attitudes, behavior that isn’t socially accepted?
– prejudice
– Racism
– Feelings toward gays & lesbians
– shoplifting

Measurement: Item Count Technique
• Here are 3 things that sometimes make people angry or upset.
After reading these, record how many of them upset you. Not which ones, just how many?
• federal govt increasing the gas tax
• professional athletes getting million dollar salaries
• large corporations polluting the environment

Measurement: Item Count Technique
• federal govt increasing the gas tax
• professional athletes getting million dollar salaries
• large corporations polluting the environment
• federal govt increasing the gas tax
• professional athletes getting million dollar salaries
• large corporations polluting the environment
• a black family moving next door

Measurement: Item Count Technique
• Randomly assign ½ of subjects to the 3 item list
• Randomly assign ½ subjects to the 4 item list
• Difference in mean # of responses between groups = % upset by sensitive item
– (mean 1 – mean 2) *100 = %

Item Count
• Control
• Non South 2.28
TREATMENT
2.24
% upset
0
• South 1.95
2.37
42
2.37 – 1.95 = 0.42 *100 = 42%

Item Count – Using poll information
• 1) The candidate graduated from a prestigious college
• 2) The candidate ran a business
• 3) The candidate’s family background
• 1) The candidate graduated from a prestigious college
• 2) The candidate ran a business
• 3) The candidate’s family background
• 4) The candidate is ahead in polls

• All 2.28
• Young
Use poll info
• Control
1.36
TREATMENT
1.39
% use poll
3.2
1.04
1.46
41
• Is it significant?
– Depends....how much does mean reflects the group? How much variation around the mean?

Central Tendency: Chapter 4
• Statistics that describe the ‘average’ or
‘typical’ value of a variable
– Mean
– Median
– Mode

Central Tendency
• Why median vs. mean?
– Household income
– Home prices

• Mean
125
92
72
126
120
99
130
100 sum=864
Central Tendency
• mean = sum X/ N
• = 864 / 8
• mean = 108
• Is this representative?

• Mean
125
92
300
126
120
99
130
100 sum=1092
Central Tendency
• mean = sum X/ N
• = 1092 / 8
• mean = 136.5
• Is this representative?

• Mean
130
126
125
120
100
99
92
Central Tendency
• median = (N +1) /2
– (8+1)/2
– 9/2
– 4.5 th
– (120, 125)
• Is this representative?

• Example
$120,00
$60,000
$40,000
$40,000
$30,000
$30,000
$30,000
Central Tendency
•
Mean = $50,000
Mdn = $40,000
Mo = $30,000
• Which is most representative?

Mean vs Median

Mean vs. Median
• median = ½ point. 12.5 = 32,000
• mean = (Sum of X’s) / n
= 816,000 / 25
= 32,640
Why is mean higher than median?

Normal vs. Skew Distributions

Which direction is skew here?

Frequency Distribution

Median vs Mean Price
• Seattle, 98121
– Median $375,900
– Mean $434,612
• Seattle, 98112
– Median $790,000
– Mean $955,750

The Distribution
• Where is mean, median, mode if
– Normal
– Left / negative skew
– Right / positive skew

Normal vs. Skew Distributions

The Distribution
• Where is mean, median, mode if
– Normal ALL THE SAME
– Left / negative skew Mean less than median
– Right / positive skew Mean greater than median

• # 6
• # 7
• #8
• #15
Chapter 4 Practice