Is Musnad Ahmed a book of Sahih Ahaadith?

Many muslims are read the Book of Ahaadith in the List of "Sihah Sittah" (The authentic six books of Ahaadith). I want to mention the names of Sihah Sittah which are as following: 1. Sahih Bukhari 2. Sahih Muslim 3. Sunan Nisaai 4. Sunan Abu Dawood 5. Sunan Ibne Majah 6. Jamia Tirmizi There is not mention the book name Musnad Ahmed which was written by Imam Ahmed bin Hanbal. Most Muslims also read and concern the Ahaadith from Masnad Ahmed. How we can prove that the Musnad Ahmed is a book of Sahih Ahaadith, if it is not a book of Sahih Ahaadith then why Muslims read and concern the Ahaadith from this book?