444 R. G. Drew Each application yielded a trace of either the resolved load (via the vector summing device) or the separate components of each resistance bridge on the pen recorder. A typical trace is shown in Fig. 3. From each trace the maximum resolved force during an application was recorded. RESULTS AND DISCUSSION Maximum application forces obtained from 840 applications were plotted in the form of two histograms (Fig. 4). 120 - •,, Ioo .s 80 o • e0 • 40 z 20 0'2 0'4 0'6 0'8 I-0 1'2 1.4 1.6 1.8 2.0 2'2 Maximum applied force (N) l*igt•e 4. Histogram of the maximum applied forces from each lipstick range during the trial. Most of the subjects applied the lipsticks with a force less than 1 N, but a 2 N force was exceeded occasionally. -- Ultra Soft .... Silver Frost. Median forces of application were 0-68 N for the Silver Frost range (10th percentile 0.46 N, 90th percentile 0.92N) and 0.73 N for the Ultra Soft range (10th percentile 0.50 N 90th percentile 1'27 N). The subjects tended to apply the Ultra Soft range using higher application forces than the Silver Frost range. A Mann-Whitney U-test showed that this difference was significant at the P 0.00001 level. Each subject's applications of the same lipstick often covered a wide range of forces (Table I). Comparison of the application loads of two lipsticks, however, revealed that within the twenty-eight subjects tested significant differences in maximum application force were found (Table II). From these differences a ranking order was compiled indicat- ing which lipsticks would be applied with a greater force: b c a y, e w v x zd. Table I. The maximum application forces observed from one subject using a single lipstick during the trial 1 0.604 0.512 0.443 0.660 0.738 2 0.667 0.438 0.708 0.676 0.851 3 0.811 0-678 0.743 0.640 0.808 4 0.397 0.604 0.682 0.996 0.705 5 0.261 0.696 0.732 0.931 0.847 6 0.642 0.774 0.942 0.782 0.829 Maximum application force (N) Application number Day 1 Day 2 Day 3 Day 4 Day 5

Mechanical stresses in lipstick 445 Table 1I. Subjects who exhibited significant differences (Mann-Whitney U-test, P 0-02) in maximum application forces between the two applied lipsticks Subjects Ultra Soft v. Silver Frost 3,4 c w 6 dw 8, 10, 11 b z 9 dz 13 c x 14 dx 16, 17 c v 21 a y 22,24 c y 23 d y 25 b y 28 a y Four independent volunteers ranked the applied lipsticks in order of colouring power: b a c e, z v, w y x d. The Kendall Rank Correlation Coefficient, x, for the ranking of applied load and subjectively assessed covering power was 0.683, indicating that individual subjects would use a lower application force to apply a lipstick with better colouring power. The subjects were divided into two age groups of approximately equal size (18-29 and 30-60 years old). Subjects over 30 years old used significantly higher application forces (Mann-Whitney U-test, P 0.025). A similar analysis of their choice of colours indicated at the P 0.025 level that the 18-29 years group were biased towards brown colours and the 30-60 years group towards the paler brown, pink or red colours. Subjects who applied lipstick frequently (daily or more often) used significantly higher application forces than those who applied lipstick less frequently. Other parameters possibly related to lipstick selection and application were also examined. The subjects' comments upon the applied lipstick was compared with the most common application force range. No overall trend was observed and subjects frequently described the same lipstick using antonyms and the reasons for lipstick selection seemed to have been many and varied. The lipstick choice combinations of a lipstick from each range revealed no bias towards particular colour combinations such as brown and dark brown, or red/pink and pale brown etc. The selection of lipstick was finally compared with subject hair colour but no trend towards any particular choice of lipstick was observed. DISCUSSION The main purpose of the work reported here was to assess the capability of the lipstick application equipment to achieve this an application trial was carried out. The equip- ment was shown to be capable of meaningful measurements and is a useful tool for assessing the force of application of lipstick. This trial was designed to assess the maximum application forces of two types of lipsticks. Subjects were expected to apply a lipstick of their own choice in a more typical