Determination of the volume density thresholds yielding the tightest correlation between the volume density and star formation. Each transparent blue line corresponds to a different value of the low-density threshold between 10 H₂ cm⁻³ and 5 × 10² H₂ cm⁻³, the solid blue plot corresponds to the optimal threshold of 2 × 10² H₂ cm⁻³. The dashed horizontal line at 0.225 dex corresponds to the spread obtained when using column densities. Values for a dense gas threshold higher than 5 × 10⁴ H₂ cm⁻³ (shaded area) are not considered because not all clouds in the sample reach such high values and the statistics thus become less and less reliable.