A demand prediction method using a computer includes calculating an error between a prediction value of an amount of a past demand and a measurement value of the amount of the past demand, the prediction value of the amount of the past demand being calculated by inputting a measurement value of a past explanatory variable to a prediction model that is constructed in accordance with a measurement value of an amount of a demand at a predetermined location and an explanatory variable that serves as an external factor that affects an increase or a decrease in the amount of the demand at the predetermined location, determining whether the calculated error is an abnormal value, acquiring a new explanatory variable if the error is determined to be the abnormal value, updating the prediction model in accordance with the acquired new explanatory variable, and newly predicting an amount of a future demand using the updated prediction model.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A demand prediction method using a computer , comprising: calculating an error between a prediction value of an amount of a past demand and a measurement value of the amount of the past demand, the prediction value of the amount of the past demand being calculated by inputting a measurement value of a past explanatory variable to a prediction model that is constructed in accordance with a measurement value of an amount of a demand at a predetermined location and an explanatory variable that serves as an external factor that affects an increase or a decrease in the amount of the demand at the predetermined location; determining whether the calculated error is an abnormal value; acquiring a new explanatory variable if the error is determined to be the abnormal value; updating the prediction model in accordance with the acquired new explanatory variable; and newly predicting an amount of a future demand using the updated prediction model, wherein at least one of the calculating, the determining, the acquiring, the updating and the newly predicting is performed by a processor included in the computer. 2. The demand prediction method according to claim 1, further comprising time-sequentially displaying the error that is determined to be the abnormal value. 3. The demand prediction method according to claim 2, wherein the new explanatory variable is acquired by receiving an input from a user. 4. The demand prediction method according to claim 2, further comprising: acquiring candidate information indicating at least one explanatory variable candidate if the error is determined to be the abnormal value; and selectively displaying the acquired candidate information. 5. The demand prediction method according to claim 4, wherein the candidate information comprises local area information concerning a local area surrounding the predetermined location in a time period when a measurement operation is performed to obtain the measurement value of the past demand that is used to calculate the error that is determined to be the abnormal value. 6. The demand prediction method according to claim 4, wherein keywords as the candidate information are acquired from an external server that provides the search engine or the social networking service in the acquiring of the candidate information if the error is determined to be the abnormal value, wherein the keywords as the candidate information are a plurality of keywords that are extracted in an order of a decreasing search count with a search engine from a larger search count to a smaller search count, or a plurality of keywords that are extracted in an order of a decreasing posting count from a larger posting count to a smaller positing count in a social networking service, and wherein, the keywords acquired as the candidate information are displayed selectively in the displaying of the acquired candidate information. 7. The demand prediction method according to claim 4, wherein at least one explanatory variable is acquired from another computer connected via a network, in the acquiring of the candidate information, if the error is determined to be the abnormal value. 8. The demand prediction method according to claim 1, further comprising: determining whether the acquired new explanatory variable satisfies a predetermined condition; updating the prediction model in accordance with the new explanatory variable if the acquired new explanatory variable is determined to satisfy the predetermined condition; and storing the new explanatory variable as a correction explanatory variable that is to correct a prediction value of a demand calculated using the prediction model, instead of updating the prediction model, if the new explanatory variable is determined not to satisfy the predetermined condition. 9. The demand prediction method according to claim 8, further comprising: calculating an affecting value on the increase or the decrease in the amount of the demand on a per correction explanatory variable basis; and correcting the prediction value in response to the calculated affecting value. 10. The demand prediction method according to claim 8, further comprising determining that the new explanatory variable does not satisfy the predetermined condition if changes in the measurement value of the new explanatory variable within a predetermined period of time are smaller in number than a predetermined count. 11. The demand prediction method according to claim 1, further comprising determining that the error is the abnormal value if the absolute value of the error is above a threshold value. 12. A demand prediction apparatus, comprising: an error calculator that calculates an error between a prediction value of an amount of a past demand and a measurement value of the amount of the past demand, the prediction value of the amount of the past demand being calculated by inputting a measurement value of a past explanatory variable to a prediction model that is constructed in accordance with a measurement value of an amount of a demand at a predetermined location and an explanatory variable that serves as an external factor that affects an increase or a decrease in the amount of the demand at the predetermined location; an explanatory variable acquirer that determines whether the error calculated by the explanatory variable calculator is an abnormal value, and acquires a new explanatory variable if the error is determined to be the abnormal value; and a demand predictor that updates the prediction model in accordance with the new explanatory variable acquired by the explanatory variable acquirer, and newly predicts an amount of a future demand using the updated prediction model, wherein at least one of the error calculator, the explanatory variable acquirer and the demand predictor includes a processor. 13. A non-transitory computer-readable recording medium storing a demand prediction program causing a computer to perform a process to predict an amount of a future demand, the process comprising: calculating an error between a prediction value of an amount of a past demand and a measurement value of the amount of the past demand, the prediction value of the amount of the past demand being calculated by inputting a measurement value of a past explanatory variable to a prediction model that is constructed in accordance with a measurement value of an amount of a demand at a predetermined location and an explanatory variable that serves as an external factor that affects an increase or a decrease in the amount of the demand at the predetermined location; determining whether the calculated error is an abnormal value, and acquiring a new explanatory variable if the error is determined to be the abnormal value; and updating the prediction model in accordance with the acquired new explanatory variable, and newly predicting an amount of a future demand using the updated prediction model.