A method and apparatus for optimization of predictor variables. The apparatus includes a value obtaining section to obtain a plurality of answers from an answer database, each answer including an answered value for each variable of a plurality of variables, the plurality of variables answered by each respondent of a plurality of respondents, a cost obtaining section to obtain a cost for answering each of the plurality of variables from a cost database, and a determining section to determine at least one predictor variable from the plurality of variables based on a prediction performance and an answering cost, the at least one predictor variable for predicting at least one remaining variable of the plurality of variables.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. An apparatus for optimization of predictor variables, comprising: a value obtaining section to obtain a plurality of answers from an answer database, wherein each answer includes an answered value for each variable of a plurality of variables, the plurality of variables answered by each respondent of a plurality of respondents; a cost obtaining section to obtain a cost for answering each of the plurality of variables from a cost database; and a determining section to determine, using a processor, at least one predictor variable from the plurality of variables based on a prediction performance and an answering cost, the at least one predictor variable for predicting at least one remaining variable of the plurality of variables. 2. The apparatus of claim 1, wherein the determining section includes: a generating section to generate a prediction model for predicting a value of at least one remaining variable candidate based on a given value of at least one predictor variable candidate; an evaluating section to evaluate the at least one predictor variable candidate using an objective function, the objective function being a function of the prediction performance and the answering cost, wherein the prediction performance is derived from the prediction model; and a selecting section to select the at least one predictor variable based on a result of evaluation of the evaluating section. 3. The apparatus of claim 2, wherein the prediction performance is a prediction accuracy, a Gini index, or an entropy gain of the prediction model. 4. The apparatus of claim 2, wherein the objective function is a function of an average prediction accuracy of the value of the at least one remaining variable candidate and the answering cost. 5. The apparatus of claim 2, further comprising: a receiving section to receive an input value of the at least one predictor variable input in an answer sheet; and a predicting section to predict a value of the at least one remaining variable by using the prediction model for predicting the value of the at least one remaining variable based on the input value of the at least one predictor variable. 6. The apparatus of claim 5, further comprising a filling section to fill in the value of the at least one remaining variable predicted by the predicting section in the answer sheet. 7. The apparatus of claim 5, wherein the receiving section is further configured to: receive an input value of the at least one remaining variable; and update the at least one remaining variable with the input value of the at least one remaining variable received by the receiving section. 8. The apparatus of claim 2, wherein: the generating section is further configured to generate a prediction model for each of the plurality of variables that has not been determined as the at least one predictor variable; the evaluating section is further configured to evaluate each of the plurality of variables that has not been determined as the at least one predictor variable; and the selecting section is further configured to select one of the plurality of variables that has not been determined as the at least one predictor variable that improves the objective function to include in the at least one predictor variable. 9. The apparatus of claim 8, further comprising: a splitting section to split a domain of the one of the plurality of variables selected by the selecting section; wherein the generating section, the evaluating section, the selecting section, and the splitting section are further configured to hierarchically select the at least one predictor variable to generate a decision tree of the at least one predictor variable. 10. The apparatus of claim 2, wherein the selecting section is further configured to stop selecting predictor variables if the objective function does not improve during evaluation by the evaluating section. 11. The apparatus of claim 1, wherein each cost is a length of time taken by one of the plurality of respondents to answer one of the plurality of variables. 12. The apparatus of claim 1, wherein the cost obtaining section is further configured to obtain the cost for answering each of the plurality of variables by using a response rate of the answered value for each of the plurality of variables. 13. The apparatus of claim 3, wherein the prediction accuracy is based on comparing a value of the at least one remaining variable predicted by the predicting section with at least one of the answered values corresponding to the remaining variable in the answer database. 14. A computer-implemented method for optimization of predictor variables, comprising: obtaining a plurality of answers from an answer database, wherein each answer includes an answered value for each variable of a plurality of variables, the plurality of variables answered by each respondent of a plurality of respondents; obtaining a cost for answering each of the plurality of variables from a cost database; and determining at least one predictor variable from the plurality of variables by a processor based on a prediction performance and an answering cost, the at least one predictor variable for predicting at least one remaining variable of the plurality of variables. 15. A computer program product comprising a non-transitory computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to perform a method comprising: obtaining a plurality of answers from an answer database, wherein each answer includes an answered value for each variable of a plurality of variables, the plurality of variables answered by each respondent of a plurality of respondents; obtaining a cost for answering each of the plurality of variables from a cost database; and determining at least one predictor variable from the plurality of variables based on a prediction performance and an answering cost, the at least one predictor variable for predicting at least one remaining variable of the plurality of variables.