正解The correct answer is ア. What this mechanism actually records is only four kinds of data: which magazine (the ID of the RFID tag), from which shelf, when it was taken out, and when it was returned. Counting the take-out events for each magazine gives the number of times it was taken off the shelf, and 〈time returned − time taken out〉 for each event gives the length of time it was taken out. Both can be calculated from the magazine ID and the time information alone, with no user information needed. Moreover, Library A does not lend magazines out and allows reading only inside the library, so the time a magazine was taken out serves as an indicator of how long it was actually being read, which directly fits Mr. B's goal of giving priority to purchasing magazines that are frequently read by users. ア stays completely within what can be obtained and is information that serves the purpose, so it is appropriate.
×Incorrectイ
The part “by user age group” cannot be obtained. The RFID tags are only attached to the magazines, and the reader on each shelf merely reads the magazine's tag; there is no way to identify who picked up a magazine. To tally by age group you would need a separate input that identifies the user, such as a membership card, and no such information exists in this mechanism. The number of times and the average time themselves can be obtained, which is exactly why the one extra condition is what makes this choice wrong.
×Incorrectウ
The part “by day of the week” can be derived from the recorded times, but “by user gender” cannot be obtained. As with イ, this mechanism does not identify users at all, so a personal attribute such as gender could never be recorded. Even if part of a choice is correct, a choice that includes an item that cannot be obtained is inappropriate.
×Incorrectエ
Neither judging whether it was “the same person” nor counting “the number of people who took it out” is possible. What is recorded is the ID on the magazine side, not an ID on the user side, so there is no way to tell whether two take-outs were made by the same person or by different people; removing duplicates and counting people are both impossible. All that can be counted is the total number of take-outs, not the number of people.
ここがポイント
The information obtainable from RFID is limited to the ID of the tagged object, the location of the reader, and the time of the reading. Attributes of objects that are not tagged (here, the users) cannot be obtained no matter how the data is processed.
When planning an information system, first list the data items that can actually be collected, then separate information that can be derived from them by calculation (counts, durations, tallies by day of the week, and so on) from information that cannot be obtained without an additional means of identification (age group, gender, actual number of people, and so on).