In order to assess quantitatively the products of combustion due to plastics, large scale tests were conducted at the Fire Research Station. The three factors investigated were crib load, weight of PVC and width of ventilation opening. Two levels of each factor were considered. The observations presented for statistical analysis were maximum temperature, maximum concentrations of carbon dioxide and carbon monoxide and minimum oxygen. Classical analysis of variance was not possible especially in view of the fact that the observations were extreme values. A suitable technique has been developed and described in this note for comparing the two levels of one factor at constant levels of all other factors. experimentation and statistical analysis. Suggestions are given for further experimentation and statistical analysis.