Contents
What does a negative correlation look like?
A negative correlation is a relationship between two variables in which an increase in one variable is associated with a decrease in the other. An example of negative correlation would be height above sea level and temperature. As you climb the mountain (increase in height) it gets colder (decrease in temperature).
What does Heteroskedasticity look like?
Heteroscedasticity produces a distinctive fan or cone shape in residual plots. Typically, the telltale pattern for heteroscedasticity is that as the fitted values increases, the variance of the residuals also increases. You can see an example of this cone shaped pattern in the residuals by fitted value plot below.
What problems does serial correlation cause?
Serial correlation causes OLS to no longer be a minimum variance estimator. 3. Serial correlation causes the estimated variances of the regression coefficients to be biased, leading to unreliable hypothesis testing. The t-statistics will actually appear to be more significant than they really are.
How to look for heteroskedasticity in a plot?
Detection of heteroskedasticity At a visual level, we can look for heteroskedasticity by examining the plot of residuals against predicted values or individual explanatory variables to see if the spread of residuals seems to depend on these variables.
When is heteroscedasticity present in a regression analysis?
When heteroscedasticity is present in a regression analysis, the results of the analysis become hard to trust. Specifically, heteroscedasticity increases the variance of the regression coefficient estimates, but the regression model doesn’t pick up on this.
Which is the best way to detect heteroscedasticity?
The simplest way to detect heteroscedasticity is with a fitted value vs. residual plot. Once you fit a regression line to a set of data, you can then create a scatterplot that shows the fitted values of the model vs. the residuals of those fitted values.
Why does heteroscedasticity result in smaller p-values?
Heteroscedasticity tends to produce p-values that are smaller than they should be. This effect occurs because heteroscedasticity increases the variance of the coefficient estimates but the OLS procedure does not detect this increase.