*21*

To fit a linear regression model in R, we can use the **lm()** function, which uses the following syntax:

model

We can then use the following syntax to use the model to predict a single value:

predict(model, newdata = new)

The following examples show how to predict a single value using fitted regression models in R.

**Example 1: Predict Using a Simple Linear Regression Model**

The following code shows how to fit a simple linear regression model in R:

#create data df frame(x=c(3, 4, 4, 5, 5, 6, 7, 8, 11, 12), y=c(22, 24, 24, 25, 25, 27, 29, 31, 32, 36)) #fit simple linear regression model model

And we can use the following code to predict the response value for a new observation:

#define new observation new frame(x=c(5)) #use the fitted model to predict the value for the new observation predict(model, newdata = new) 1 25.36364

The model predicts that this new observation will have a response value of **25.36364**.

**Example 2: Predict Using a Multiple Linear Regression Model**

The following code shows how to fit a multiple linear regression model in R:

#create data df frame(x1=c(3, 4, 4, 5, 5, 6, 7, 8, 11, 12), x2=c(6, 6, 7, 7, 8, 9, 11, 13, 14, 14), y=c(22, 24, 24, 25, 25, 27, 29, 31, 32, 36)) #fit multiple linear regression model model

And we can use the following code to predict the response value for a new observation:

#define new observation new #use the fitted model to predict the value for the new observation predict(model, newdata = new) 1 26.17073

The model predicts that this new observation will have a response value of **26.17073**.

**Potential Errors with Predicting New Values**

The most common error you may run into when attempting to predict a new value is when **the dataset you used to fit the regression model does not have the same column names as the new observation youâ€™re attempting to predict**.

For example, suppose we fit the following multiple linear regression model in R:

#create data df frame(x1=c(3, 4, 4, 5, 5, 6, 7, 8, 11, 12), x2=c(6, 6, 7, 7, 8, 9, 11, 13, 14, 14), y=c(22, 24, 24, 25, 25, 27, 29, 31, 32, 36)) #fit multiple linear regression model model

Then suppose we attempt to use the model to predict the response value for this new observation:

#define new observation new #use the fitted model to predict the value for the new observation predict(model, newdata = new) Error in eval(predvars, data, env) : object 'x1' not found

We received an error because the column names for the new observation (x_1, x_2) do not match the column names of the original data frame (x1, x2) we used to fit the regression model.

**Additional Resources**

How to Perform Simple Linear Regression in R

How to Perform Multiple Linear Regression in R

How to Create a Residual Plot in R