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    Question

    A time series dataset shows monthly sales data for a

    retail store over the last three years. After performing decomposition, you observe that the residual component exhibits no clear pattern and appears to be randomly distributed. What does this imply about the quality of the decomposition?
    A The model has overfitted the data, and the residuals should show some structure. Correct Answer Incorrect Answer
    B The decomposition is likely correct, as residuals should ideally be random and patternless. Correct Answer Incorrect Answer
    C The residuals should be removed entirely for the decomposition to be valid. Correct Answer Incorrect Answer
    D The decomposition might be incorrect, as residuals should reflect seasonality or trend. Correct Answer Incorrect Answer
    E The model is not appropriate for this data, and other models should be tested. Correct Answer Incorrect Answer

    Solution

    In time series analysis, the decomposition process breaks down the data into three components: trend, seasonality, and residuals. The residuals represent the random noise left after removing the trend and seasonality components. Ideally, the residuals should not show any patterns or trends, indicating that the model has captured all systematic information from the data. If the residuals are random, it suggests that the model has successfully accounted for both trend and seasonality. Hence, the fact that the residuals are randomly distributed indicates that the decomposition process is likely correct. Why Other Options Are Incorrect: • A: Overfitting refers to a model that captures too much noise in the data, not leaving random residuals. Overfitting typically results in residuals that show patterns, not randomness. • C: Residuals are an essential part of decomposition and should not be removed. They represent the noise or error in the model and are necessary for validating the model's fit. • D: The residuals should not reflect seasonality or trend, as these components are already removed during the decomposition process. Any structure in the residuals would indicate that the decomposition model has not fully captured the data's underlying patterns. • E: A model showing random residuals indicates that it has captured the key patterns in the data, and there is no immediate need to test other models.

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