294 Index Q Quantification, 185 Quantitative data, 273, 274 Questionnaire for User Interaction Satisfaction (QUIS), 188­189, 210 Questionnaires for assessing websites, 221­225 data, 273 Quick-and-dirty usability scale, 198­199 QUIS, see Questionnaire for User Interaction Satisfaction R Randomness, 10­12 Rating scales, 15, 26, 63­74 Ratio, 242 Realistic usability testing, 112 Rejection regions, 248, 249 Reliability, 187 ASQ, measurements of, 213 of SUMI scales, 191 Replicability, 185 Representativeness, 10­12 Return on investment (ROI), 158, 158 Robustness, quantitative definition of, 174 Software Usability Measurement Inventory (SUMI), 190­191 Software, usability problems in, 167 Software Usability Scale (SUS), 186­187, 198­210 alternate form of, 204­210 norms, 200­204 and NPS, 229­230 Split-tailed test, 250 Standard deviation, 276 and mean, 278 Standard error of the mean (SEM), 50, 282­283 Standard Wald formula, 126, 132­133 interval, 124 Standardized questionnaires, 185 assessing quality of, 187 Standardized Universal Percentile Rank Questionnaire (SUPR-Q), 223­224 Standardized usability questionnaires, 185­187 Statistical decision making, 288 Statistical test, 5, 93 Statistics, 19 Stevens' Power Law, 217 Stevens, S.S., 242­243, 246 Stratified sampling, 11