Statistics: Correlation and Scatterplots

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Questions and Answers

What does a correlation coefficient (r) of zero indicate about the relationship between two variables?

There is a perfect negative correlation between the variables.

There is a perfect positive correlation between the variables.

There is a strong positive correlation between the variables.

There is no association between the variables. (correct)

In a scatterplot, what does a negative correlation suggest about the direction of the relationship between two variables?

The changes in the two variables are constant and predictable.

There is no relationship between the two variables.

As one variable increases, the other variable also increases.

As one variable increases, the other variable decreases. (correct)

Which of the following statements best describes a correlation coefficient (r) of +1?

There is a weak positive relationship between the variables.

There is a moderate negative relationship between the variables.

There is no relationship between the variables.

There is a strong positive relationship between the variables. (correct)

When interpreting relationships in a scatterplot, which aspect refers to how closely the points cluster around a line?

Degree Signup and view all the answers

What is the implication of a positive association in a correlation?

Increases in one variable are accompanied by increases in the other. Signup and view all the answers

What is the minimum correlation coefficient required for significance with 2 degrees of freedom?

.95 Signup and view all the answers

Why is significance more sensitive when dealing with smaller sample sizes?

Larger sample sizes lead to more stable correlations. Signup and view all the answers

In testing for an association between age and personality traits, what indicates a significant result?

A significant p-value less than .05 Signup and view all the answers

What alternative correlation coefficient is derived from ranking data?

Spearman’s rho Signup and view all the answers

Which of the following statements is true regarding the correlation coefficient?

The correlation coefficient can only range from -1 to +1. Signup and view all the answers

What is a potential outcome of a small sample size in correlation studies?

Inflated correlation values Signup and view all the answers

What does a significant correlation imply about the relationship between two variables?

It suggests a meaningful relationship worth further investigation. Signup and view all the answers

How does the calculation of Spearman’s correlation differ from Pearson’s correlation?

Spearman is used for ordinal data, while Pearson is for interval/ratio data. Signup and view all the answers

What does a correlation coefficient (r) value of 0.76 indicate about the association between two variables?

There is a strong positive relationship between the variables. Signup and view all the answers

In the calculation of the correlation coefficient (r), what does covariability represent?

The extent to which two variables change together. Signup and view all the answers

When calculating variability for variable X, which formula component is used?

Sum of squares deviations from the mean of X. Signup and view all the answers

What does a scatterplot visually represent in relation to two variables?

The linearity and strength of their association. Signup and view all the answers

In the context of null hypothesis significance testing, what does a binary Yes/No answer typically indicate?

Whether the evidence supports a specific hypothesis. Signup and view all the answers

If the sum of products of deviations for both variables is zero, what can be concluded about their correlation coefficient (r)?

There is no relationship between the variables. Signup and view all the answers

How does the correlation coefficient change if the sum of squares for one variable becomes very large while the sum of products remains constant?

The correlation coefficient will decrease. Signup and view all the answers

What implication does a correlation coefficient (r) of 1 have regarding the covariance of two variables?

Covariance equals the variability of both variables. Signup and view all the answers

Study Notes

Scatterplots

Visualize the association between two variables (X and Y)
Plot pairs of X-Y scores on a graph with X-axis and Y-axis

Correlation

Quantifies the association between two variables (X, Y)
Uses a correlation coefficient (r)
Measures the direction and degree of a linear relationship between variables
Can be positive, negative, or zero, showing the direction of association

Direction, Degree, and Form of Association

Direction:
- Positive: Increases in X lead to increases in Y
- Negative: Increases in X lead to decreases in Y
- No relationship: Knowing X provides no information about Y
Degree:
- Correlation coefficients (r) range from -1 to 1
- r = 1 or -1 indicates a perfect correlation (strongest)
- r = 0 indicates no association (weakest)
Form:
- Correlation measures the strength of a linear relationship
- Other forms of association (non-linear) may exist

Calculating the Correlation Coefficient (r)

Covariability of X and Y divided by the variability of X and Y separately:
- Variability is measured using the sum of squares (SS) for each variable
- Covariability is measured using the sum of products (SP)
r = SP / √(SSx * SSy)
Represents the degree to which X and Y change together

Null Hypothesis Significance Testing

Used to determine if an observed correlation is statistically significant
Examines the probability of obtaining the observed correlation by chance
Requires a p-value to determine if there is enough evidence to reject the null hypothesis
Significance depends on the degrees of freedom (df), which is influenced by sample size
- Larger df = larger sample = More robust results
- Smaller df = Smaller sample = Less robust results

Why Significance Depends on Sample Size

Correlation estimates become more stable as sample size increases
At smaller sample sizes, large correlations may be due to chance
A small sample size creates more vulnerability to chance events

Example Correlation Matrix

Contains correlations between multiple variables, presented in a table
Shows the strength and direction of relationships between variables

Spearman's Correlation

Alternative to Pearson's correlation coefficient (r)
Uses ranks of data points instead of raw scores
Suitable when data is not normally distributed or when there are outliers

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Description

This quiz covers the concepts of correlation and scatterplots in statistics. You will learn how to visualize associations between two variables and calculate the correlation coefficient to measure the relationship's direction and degree. Test your understanding of these fundamental statistical tools.