Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The limits of detection for superconducting quantum interference devices (SQUID), optical pumped magnetometer (OPM), Fluxgate, and magneto-impedance (MI) sensors at 10 Hz are approximately 1 fT/ √ Hz [1], 10 fT/ √ Hz [2], 5 pT/ √ Hz [3], and 10 pT/ √ Hz [4], respectively.
Category: