When computing VaR using parametric methods, a common implicit assumption is that both the mean and standard deviation of returns are constant over time.
* Constant Mean: The expected return of the asset or portfolio does not change over the time period considered.
* Constant Standard Deviation: The volatility of returns, which measures the dispersion of returns from the mean, is assumed to be constant.
This assumption simplifies the calculations as it allows the use of historical data to estimate future risks.
However, it may not always hold true in real-world scenarios where markets can exhibit changing volatility and return patterns.
References
* How Finance Works.pdf, p. 201