Please answer a question about the following article about Human Development Index:

A new index was released on December 18, 2008. This so-called "statistical update" covered the period up to 2006 and was published without an accompanying Human Development Report. The update is relevant due to newly released estimates of purchasing power parities (PPP), implying substantial adjustments for many countries, resulting in changes in HDI values and, in many cases, HDI ranks.

What period was covered by the statistical update index released in December of 2008?
up to 2006