What is the basic measure of ex-post risk?

What is the basic measure of ex-post risk? 

Using historic returns as a measure of future risk has been a traditional method used by investors to determine the riskiness of a given asset. Ex-post risk is often used in value at risk analysis – a tool used to give investors a best estimate of the maximum amount of loss that they could expect to incur on any given trading day. It is a type of risk measurement technique that uses historic returns to predict the riskiness of a certain investment in the future. This type of risk measure is the equivalent of the statistical variance of an asset’s returns relative to its mean.