Shows the relationship between two different sets of scores
Answers the question: "Are high scores on one variable (thing) predictive of high scores on some other measure (or realm)?"
Shows the degree to which two things change together (e.g., height
& weight; distance from favorite person & happiness;
It's "going-togetherness"
Degree of correspondence between two values
The tendency of one score to vary or change when another one
does so.
Is there a correlation (relationship) between (from birth) Physical growth and knowledge of the world? (Yes, as you grow with age, you tend to get smarter.)
The amount you drink and how good the other person in the room looks?
(Yes.
Many pub goers have reported that alcohol can make others look more attractive)
Level of literacy and number of books read? (The more
books you read, the more literate you become)
Does a strong correlation mean that a change in one thing causes the
other thing to change?
(Correlation is NOT causation.
There is just a "tendency" of the two to occur
together.)
A high correlation between two sets of scores can be used to prove that two tests measure the same thing
The Smith Test of Intelligence
The Jones Test of Intelligence
Rank order (among a group who took test together)
Rank order
Student 1: 1st
Student 1: 1st
Student 2: 4th
Student 2: 3th
Student 3: 9th
Student 3: 9th
Student 4: 7th
Student 4: 7th
Student 5: 6th
Student 5: 6th
Student 6: 2nd
Student 6: 2nd
Student 7: 5th
Student 7: 5th
Student 8: 3rd
Student 8: 4rd
Student 9: 8th
Student 9: 8th
More on Correlation
Shows the relationship between two different sets of scores
Answers the question: "Are high scores on one variable (thing) predictive of high scores on some other measure (or realm)?"
Shows the degree to which two things change together
(e.g., height & weight, distance from favorite person &
happiness,
Northerness in USA (between Mississippi and Atlantic Ocean and
educational level)
It's "going-togetherness"
Degree of correspondence between two values
The tendency of one score to vary or change when another one
does so.
Is there a correlation (relationship) between
(from birth) Physical growth and knowledge of the world?
(Yes. As people grow, they tend to gain more information
about the world)
The amount you drink and how good the other person in the room looks?
(So they say.)
Level of literacy and number of books read?
(Yep. One may not cause the other, but we certainly see
this trend.)
Does a strong correlation mean that a change in one thing causes the
other thing to change?
(Correlation is NOT causation. There is just a
"tendency" of the two to occur together.)
A high correlation between two sets of scores can be used to prove that two tests measure the same thing
The Smith Test of Intelligence
The Jones Test of Intelligence
Rank order (among a group who took test together) Rank
order
Student 1: 1st Student 1:
1st
Student 2: 4th Student 2:
3th
Student 3: 9th Student 3:
9th
Student 4: 7th Student 4:
7th
Student 5: 6th Student 5:
6th
Student 6: 2nd Student 6:
2nd
Student 7: 5th Student 7:
5th
Student 8: 3rd Student 8:
4rd
Student 9: 8th Student 9:
8th
Quiz on Norms and Correlations
1. List three things that must be considered in determining if the "norms"
(data on comparison groups) provided by a test are useful.
2. You are told that there is a correlation coefficient of +1.50
What can you tell from this information?
3. Which is the strongest connection/correlation coefficient?
+.46
+.93
-.1
-.96
4. The "line of best fit" might also be called the _____________ line.
5. Most alcoholics drank milk as children. The correlation between
the two
traits is very strong. Therefore drinking milk places one at
risk for
alcoholism. Right? Most heroin addicts smoked marijuana
previously.
Marijuana use causes future heroin use. Right?
(Nope: Correlation does not prove causation)
Tom Mcintyre at www.BehaviorAdvisor.com