There is an inadequate attempt to make a decision on the accuracy and reliability of the effectively usable dosage rates of sewage sludge ash (SSA) for replacement and improvement of coarse-grained soil. This study introduces an investigation on the effectively usable dosages of SSA for subbase construction by using factorial experimental approach and effect size estimation. California bearing ratio (CBR) tests have been carried out to measure the parameters of CBR value, energy absorption capacity (E-d) and ultimate bearing capacity (q(u)) of the soil + SSA mixtures for which proportions were 0, 5, 10, 15, 20, 30, 40, 50 and 60% by dry weight of the mixture. The effectively usable dosages of SSA for both improvement and substitute in highway construction, based on the foregoing parameters, have been assessed essentially using multiple comparison tests and effect size parameters (omega-squared, eta-squared and Cohen's d). The results of statistical analysis demonstrate that while there is an insignificantly variation (increase or decrease) (p > .05 and Cohen's d <= .41) at the dosage rates of 5 and 15% for CBR, 15% for E-d and 5, 20, 30 and 40% for q(u), there is a significant decrease (p <= .05 and Cohen's d > 2.7) in the values of CBR, E-d and q(u) due to the addition of SSA at the dosages of 50, 50 and 60%, respectively. Accordingly, it is suggested that SSA be introduced into sandy soil for improvement or replacement at the dosage rates less than 50% in the stabilisation applications. From the statistical analysis performed here, it is also found that multiple comparison tests and effect size estimates compare well in the decision-making.