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NAs
Factors
Summary

## Instruction

Great! You probably noticed that some of the yard_area values were marked as NA.

In R, NA represents a missing value. These may be values that someone forgot to enter, or information that is simply unavailable. Missing values affect the quality of your data. A variable is not very meaningful when most of its values are missing.

## Exercise

Why can missing values be problematic? Try calculating the average of the houses\$yard_area column. Observe the result.

### Stuck? Here's a hint!

You have to use the mean() function.