A mood score calculation system includes a processor and a memory. The memory is configured to hold relationship information on a relationship between a mood score of a user and a statistical feature for indicating fluctuation of operational interval time of a user terminal. The processor is configured to acquire an operation log of a first user terminal. The processor is configured to calculate, from the operation log, a value of a statistical feature for indicating fluctuation of operational interval time of the first user terminal. The processor is configured to determine a mood score of a user of the first user terminal based on the value of the statistical feature and the relationship information and output the mood score.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A mood score calculation system, comprising: a processor; and a memory, the memory being configured to hold relationship information on a relationship between a mood score of a user and a statistical feature for indicating fluctuation of operational interval time of a user terminal, the processor being configured to: acquire an operation log of a first user terminal; calculate, from the operation log, a value of a statistical feature for indicating fluctuation of operational interval time of the first user terminal; determine a mood score of a user of the first user terminal based on the value of the statistical feature and the relationship information; and output the mood score. 2. The mood score calculation system according to claim 1, wherein the statistical feature comprises a fractal dimension for characterizing the operational interval time of the user terminal. 3. The mood score calculation system according to claim 1, wherein the statistical feature comprises a scaling-law index for characterizing a cumulative probability distribution of the operational interval time of the user terminal. 4. The mood score calculation system according to claim 1, wherein the processor is configured to determine the mood score as an error value when a total number of operations, based on which the value of the statistical feature exhibited by the operation log is calculated, is smaller than a threshold value. 5. The mood score calculation system according to claim 2, wherein the processor is configured to: calculate a coefficient of determination between the operation log and an approximation equation used for calculating the fractal dimension; compare the coefficient of determination with a threshold value; and determine the mood score as an abnormal value when the coefficient of determination is smaller than the threshold value. 6. The mood score calculation system according to claim 3, wherein the processor is configured to: calculate a coefficient of determination between the operation log and an approximation equation used for calculating the scaling-law index; compare the coefficient of determination with a threshold value; and determine the mood score as an abnormal value when the coefficient of determination is smaller than the threshold value. 7. The mood score calculation system according to claim 1, wherein the relationship information indicates a relationship between a variation of the calculated statistical feature from a reference value and the mood score, and wherein the processor is configured to: calculate the reference value using a number of operations in the operation log; and determine the mood score based on the variation of the calculated statistical feature from the calculated reference value and the relationship information. 8. The mood score calculation system according to claim 1, wherein the operation log comprises an operation log of a first input device and an operation log of a second input device, and wherein the processor is configured to: determine a first mood score from the operation log of the first input device and a second mood score from the operation log of the second input device; and output the first mood score and the second mood score. 9. A method of determining a mood score, which is performed by a computer system configured to hold relationship information on a relationship between a mood score of a user and a statistical feature for indicating fluctuation of operational interval time of a user terminal, the method comprising: acquiring, by the computer system, an operation log of a first user terminal; calculating, by the computer system, from the operation log, a value of a statistical feature for indicating fluctuation of operational interval time of the first user terminal; determining, by the computer system, a mood score of a user of the first user terminal based on the value of the statistical feature and the relationship information; and outputting, by the computer system, the mood score.