You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Human_Development_Index

Context:
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.

Question:
What period was covered by the statistical update index released in December of 2008?

up to 2006