Decide What to Retain or Suppress
Filters can reduce noise or isolate a period range of interest. Their type, order, passband, window length, and implementation are chosen for the purpose. The resulting waveform reflects those choices.
Check Time Resolution and Gaps
Short-period components such as Pc3 and longer daily variations require different time resolutions. Sampling intervals, interpolation, and resampling need to be checked. Comparisons between datasets with different resolutions also need an explanation of their limits.
Read Edges and Gaps with Care
Processing near record edges or gaps can emphasize features not present in the original observation. Compare with unprocessed data and check edge handling and excluded intervals. Important features should be examined under alternative settings to see whether the conclusion depends strongly on a particular choice.
Read the Settings Accompanying a Figure
For a filtered figure, check the target band, baseline interval, detrending, interpolation, and excluded sections. The reason for choosing the processing, and conditions where it is unsuitable, also help explain the result.