*25*

In statistics, simple linear regression is a technique we can use to quantify the relationship between a predictor variable, x, and a response variable, y.

When we conduct a simple linear regression, we obtain a “line of best fit” that describes the relationship between x and y, which can be written as:

ŷ = b_{0} + b_{1}x

where:

- ŷ is the predicted value of the response variable
- b
_{0}is the y-intercept - b
_{1}is the regression coefficient - x is the value of the predictor variable

Sometimes we’re interested in using this line of best fit to construct a **prediction interval **for a given value of x_{0}, which is an interval around the predicted value ŷ_{0} such that there is a 95% probability that the real value of y in the population corresponding to x_{0} is within this interval.

The formula to calculate the prediction interval for a given value x_{0} is written as:

ŷ_{0} +/- t_{α/2,df=n-2} * s.e.

where:

s.e. = S_{yx}√(1 + 1/n + (x_{0} – x)^{2}/SS_{x})

The formula might look a bit intimidating, but it’s actually straightforward to calculate in Excel. Next, we’ll walk through an example of how to use this formula to calculate a prediction interval for a given value in Excel.

**Example: How to Construct a Prediction Interval in Excel**

The following dataset shows the number of hours studied along with the exam score received by 15 different students:

Suppose we would like to create a 95% prediction interval for the value x_{0} = 3. That is, we want to create an interval such that there is a 95% probability that the exam score is within this interval for a student who studies for 3 hours.

The following screenshot shows how to calculate all of the necessary values to obtain this prediction interval.

**Note:** The formulas in column *F *show how the values in column *E *were calculated.

The 95% prediction interval for a value of x_{0} = 3 is **(74.64, 86.90)**. That is, we predict with 95% probability that a student who studies for 3 hours will earn a score between 74.64 and 86.90.

A couple notes on the calculations used:

- To calculate the t-critical value of t
_{α/2,df=n-2}we used α/2 = .05/2 = 0.25 since we wanted a 95% prediction interval. Note that higher prediction intervals (e.g. 99% prediction interval) will lead to wider intervals. Conversely, a lower prediction interval (e.g. 90% prediction interval) will lead to a more narrow interval. - We used the formula
**=FORECAST()**to obtain the predicted value for ŷ_{0 }but the formula**=FORECAST.LINEAR()**will return the exact same value.