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Panel (a) The split-plot structure for each gas with a general Whole-Unit (WU) and the related Sub-Unit (SU). (l = 1, ..., 8; t = 1, ..., 4). Panel (b) shows the WP structure with the 6 WUs, with each WU split into 8 SUs. Gc: gas concentration; Hum: humidity; Wt: working temperature; Mt: material type.
