Schematic bar plot of HCOOCH₃/CH₂OHCHO (top) and HCOOCH₃/(CH₂OH)₂ (bottom) against L_bol. The sources from Table 4 have been plotted in descending order of luminosity from left to right on the x-axis, but not to scale, and the sources have been grouped into high-mass sources (blue), intermediate-mass protostar (red) and low-mass protostars (green). The white bars for four of the high-mass sources are upper or lower levels, which is indicated by the direction of the arrow. The luminosity for the top plot spans from Sgr B2(N) with L_bol = 10⁷L_⊙ to IRAS NGC 1333 4A with L_bol = 7.7 L_⊙, while it spans from W51/e2 with L_bol = 4.7 × 10⁶L_⊙ to IRAS NGC 1333 4A with L_bol = 20 L_⊙ in the bottom plot.