A company has developed a linear trend model to forecast monthly sales. The following data show the actual sales and the "fitted" sales for months 1-12. 
Based on these data, what is the value for the mean absolute deviation for months 1-12?
Definitions:
Token Economy
A behavior modification system that utilizes tokens as reinforcement for desired behaviors, which can later be exchanged for privileges or rewards.
Schizophrenia Patient
A schizophrenia patient is an individual diagnosed with schizophrenia, a mental disorder marked by distorted thinking, perceptions, emotions, language, sense of self, and behavior.
Socially Active
Engaging in and contributing to community or social group activities, showing involvement with others in public contexts.
Operant Conditioning
A learning process by which the strength of a behavior is modified by reinforcement or punishment.
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