Assume that the values of the assets in the portfolio experience the same percentage change as they did during one of the days in a historical reference period.

Assume that a group of market variables has a multivariate normal distribution based on their movements during a historical reference period, and use a sampled value from this distribution to calculate asset values.

Assume that the values of the assets in the portfolio have a multivariate normal distribution based on their movements during a historical reference period, and then sample once from this distribution of asset values.