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The **symmetric mean absolute percentage error (SMAPE) **is used to measure the predictive accuracy of models. It is calculated as:

**SMAPE** = (1/n) * Σ(|forecast – actual| / ((|actual| + |forecast|)/2) * 100

where:

**Σ**– a symbol that means “sum”**n**– sample size**actual**– the actual data value**forecast**– the forecasted data value

The smaller the value for SMAPE, the better the predictive accuracy of a given model.

This tutorial explains two different methods you can use to calculate SMAPE in R.

**Method 1: Use smape() from the Metrics Package**

One way to calculate SMAPE in R is to use the **smape()** function from the **Metrics** package:

library(Metrics) #define actual values actual #define forecasted values forecast #calculate SMAPE smape(actual, forecast) [1] 0.1245302

We can see that the symmetric mean absolute percentage error for this model is **12.45%**.

**Method 2: Write Your Own Function**

Another way to calculate SMAPE is to create our own function as follows:

find_smape function(a, f) { return (1/length(a) * sum(2*abs(f-a) / (abs(a)+abs(f))*100)) }

We can then use this function to calculate the SMAPE between a vector of actual values and forecasted values:

#define actual values actual #define forecasted values forecast #calculate SMAPE find_smape(actual, forecast) [1] 12.45302

Once again the SMAPE turns out to be **12.45%**, which matches the results from the previous example.

**Additional Resources**

How to Calculate MAPE in R

How to Calculate MAD in R

How to Calculate MAE in R

How to Calculate RMSE in R

How to Calculate MSE in R