PSY CHO-RHEOLOGY 2 5 3 10 5 'E 10 3 • i0 • x/'•sc o•½•J'cp ] Figure 3. The pictorial representation of texture using a viscosity, elasticity, per cent of quickly recoverable strain diagram. SPECIFICATION OF DESIRABLE RHEOLOGICAL PROPERTIES Having set up methods for describing the texture of a product by means of a set of numbers, their optimum values must be established, and the allowable deviations from the optimum which do not diminish the product's acceptibility. Sensory evaluation of the products and correlation of these results with the results of physical tests is the route to this goal. SETTING UP A PANEL In selecting a panel there are two choices as to the type of personnel to be used they may be non-expert, typical consumers, or a trained, expert group. The panel chosen will depend on availability, time and cost con- siderations, but also on the use to be made of the data. The objectives are, the following, (i) To establish that a panel of judges can assess the physical properties to be measured.

254 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 10 5 IOO Figure 4. The pictorial representation of hard, firm, soft and pourable materials. (ii) Having completed (i) above, to use the panel to determine the optimum for each physical property, and the acceptable limits of variation for each one. It sounds very simple. In fact it is a little more difficult than it sounds, because of the large variation in sensory judgments between people. It is never possible to obtain an absolute answer from a sensory test. The best that can be achieved is a result which is 'statistically significant', at the 5•o, 1 •o, or possibly even 0.1 •o confidence level, depending on the number of people used on the panel, and how consistent they are in their judgments. It should be obvious from the preceding paragraph that a well-trained panel, carefully selected for their sensitivity, reproducibility and interest in participation will take much longer to set up but provide much more valuable information, and at a greater rate, once established. TYPES OF PANEL TEST The organization of panel tests, and the coding and presentation of