2.6/2.7: Correlations Flashcards
What are z scores used for?
To compare statistics from different studies that may have different units being compared.
What are correlational designs referred to as?
Observational
What do correlational designs measure?
Only measurable variables
What do correlational deigns occur with?
Association claims
What is the goal of correlational designs?
To detect relationships in variables as they naturally occur.
What are the 3 characteristics of Pearson’s correlation statistic?
- Both variables are at least interval (continuous)
- Both variables are normally distributed in the population
- The association can be summarized with a straight line (linear)
Correlation Statistic
Describes the strength and direction of the relationship between two quantitative variables.
What are correlation statistics only used for?
Linear relationships that can be modeled with a straight line
How is the strength of correlation visually estimated?
How close the points are to the trend line.
What is the range for strength of correlation?
-1 to 1
How is the direction of a linear model shown?
By the slope (pos/neg)
“r”
A unitless descriptive statistic
What does r not relfect
Original units of the variables
What is true about effect studies?
They are comparable across different variables and studies
What is the formula for “r”
(Z score of X * Z score of Y) / (N-1)
Association Claims
Assert that two variables are related to each other
What is true about the frequency in association claims?
The frequency of one variable is tied to the frequency of another.
What does the Pearson correlation statistic support?
Association claims
What do extreme values in a data set have an impact on?
The correlation value of the data set
What can outliers do to the correlation value?
Increase, mute, or decrease the correlation
When there is no correlation between variables, what value is the correlation around?
0 - Not exactly 0
How are strong correlations presented in the graph?
Tightly bound to the line of slope
What correlation score do vertical lines have?
Exactly 0
What do vertical lines have a correlation score of exactly 0?
A relationship cannot be determined due to the same x value having multiple y values
What 2 ways help you protect your data set from outliers?
- Check one variable distribution for the present outliers first (identify)
- Look at the scatter plots before interpreting the correlation value (identify outlier influence)
What do Z Scores tell us?
How many units a value variates from the mean, in terms of standard deviation.
What happens to the z score of a value when the value is below the mean?
It will be negative
How do you calculate the z score of a value?
- Find variation from the mean
- Divide the variation by the standard deviation
How do outliers in the same direction of the slope affect the correlation?
Increases the strength of the relationship (more + or -)
How do outliers in the opposite direction of the slope affect the correlation?
Decreases the strength of the relationship (closer to 0)
What effect does taking out extreme values have on the correlation value of the data set?
Increases the correlation (stronger)
What should you do if the outlier is proved to be a mistake in the data?
- Remove the value
- Report the removal in the methods section; explain what impact it had on the data / why it was an outlier.
