****SPOILER *****Michael Learned's abscence

She was not challenged by the role - she said she was tired of pouring coffee in the kitchen. She was not in the last half of season 7; in return, she agreed to return for the first half of season 8 to fulfill her contract.

Sadly, the show was never really the same after that. The show declined when Richard Thomas left, but it was at least believable that the eldest son left home. Tried as they might, they could never really explain Olivia's absence in a way that wasn't contrived ("Oh, she has tuberculosis and must recover at Aunt Kate's house". "Oh, now she's going to volunteer in the Red Cross during the war". "Uh-oh, the tuberculosis is back - she needs to move to Arizona so she can recover in a dry climate.").