STATISTICAL APPROACH TO COMMON VARIABLES 445 .25 .50 .75 1.00 1.25 1.50 L75 2.00 2.25 2.50 2.75 5.00 5.25 5.50 5.75 4.00 4.25 4.50 Initiel Stability Figure I--Differences between levels- all factors i showed significant dependence on the preparative variables. As an ex- •ample, the significant differences found for the factor of temperature are indicated in Table 7. Similar organization of the data was made for each I preparative factor. In addition to the criterion of a difference between [levels of 2.00 or greater, the practical criterion of an indication of stability beyond that measured initially was imposed. For a given emulsifier-oil pair, it was decided that a factor was significant only if significant differ- •ence values appeared for at least the initial and one-day stability or for 'the one-day and one-week stability observations. If a significant 'difference appeared at only one time period but not at the other two, the If actor in question was not considered to be important in contributing to i the stability of the emulsion obtained from the oil-emulsifier pair in lquestion. Stability of the o/w emulsions formed was greater than that of the w/o ,emulsions. As had been expected, it was found that the nature of the oil •and the nature of the surfactant are of prime importance in determining Ithe stability of the resulting emulsions. Indeed, of the 312 emulsifier-oil- Ivariable comparisons of the 90-30 groups which were made for o/w emuL isions, 226 showed no significant dependence of stability on any individual 'preparative factor by the criteria used in this analysis. Thus the results •of this study have reaffirmed the view that the procedure used is often ot7 isecondary importance in emulsion stability. The relation of chemical ,constitution and physical characteristics of oil and emulsifier to stability ,and other emulsion characteristics is under active investigation in this Ilaboratory (2, 3, 7). The 13 emulsifiers used in this investigation represent a variety of chemical types and hydrophile-lipophile balances. Similarly, Ithe four oils investigated are of different chemical types.

446 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS