Distribution of the reduced enhancement η (Eq. 8) of the DIB normalised equivalent width, EN, in all 25 APO sight lines. In each histogram, red arrows mark the position of η-δ = ±2, where δ is the shift of the central roughly Gaussian distribution from EN_av (Table A.1). The line of sight of HD 175156 has four DIBs with formal values of η₁₇₅ − 14, 15, 17 and 35 (Table C.1) − outside of the range displayed here (−3 < η <10.5). The line of sight of HD 148579 has four DIBs with formal values of η₁₄₈ − 11, 13, 16 and 32 (Table C.2) − outside of this range.