The probability density function \(f(x)\) of a random variable \(X\) which takes real values is
\[ f(x) = \frac{1}{3\sqrt{2\pi}} \exp\left(-\frac{x^2}{18}\right), \quad x \in (-\infty, +\infty) \]
Which one of the following statements is correct about the random variable \(X\)?