Multidimensional scaling Research Methods Fall 2010 Tamás Bőhm Multidimensional scaling (MDS) • Earlier methods: measuring the properties of one specific perceptual dimension (e.g. brightness, pitch) – Simple stimuli with one physical dimension varied Spot of light, pure tones etc. • MDS: exploring what the perceptual dimensions are – Complex stimuli with multiple dimensions Faces, melodies, etc. • Perceptual maps are created from similarity judgments Multidimensional scaling • What does the MDS algorithm do? From a matrix of distances… Kruskal & Wish, 1978 Multidimensional scaling • What does the MDS algorithm do? …it calculates a map… Multidimensional scaling • What does the MDS algorithm do? …but it cannot tell the orientation and the meaning of the axes. Multidimensional scaling Experiment setup 1. Present the stimuli pair-wise and ask the observer how similar they are (e.g. on a 0-100 scale) 2. Create the dissimilarity matrix 3. Run MDS to get a perceptual map of the stimuli 4. Interpret the dimensions of the map Multidimensional scaling • Stimuli: 4 different salt concentrations (A: 0.5%, B: 2%, C: 1%, D: 1.5%) 1. Dissimilarity judgments (0: perfect similarity; 100: no similarity) A vs B: 90 A vs C: 10 A vs D: 55 B vs C: 80 B vs D: 35 C vs D: 45 2. Dissimilarity matrix A A B C D 90 10 55 B 90 80 35 C 10 80 D 55 35 45 45 Symmetrical (i.e. A vs B = B vs A) Multidimensional scaling 3. Perceptual map: each stimuli represents a point, their distances correspond to dissimilarities A C D 1D solution A A B 90 C D 10 55 B B 90 80 35 C 10 80 D 55 35 45 45 Multidimensional scaling 4. Interpreting the dimensions: looking for correspondences between physical and perceptual dimensions B D Dimension 1 (from MDS) C A Salt concentration Dimension 1: intensity of salt taste Multidimensional scaling Another example: soft drinks Coke Diet Coke Pepsi Diet Pepsi Cherry Diet Coke Cherry Coke Coke Diet Coke 20 Pepsi 10 22 Diet Pepsi 22 10 20 Cherry Coke 30 36 40 45 Diet Cherry Coke 36 30 45 40 20 Multidimensional scaling Diet taste Pepsi 20 10 Coke 10 20 30 Cherry Coke Cherry taste Diet Pepsi Diet Coke 30 20 Diet Cherry Coke 2D solution Multidimensional scaling Shepard, 1963: • Morse-codes presented in pairs to naïve observers (each possible combination) • Same/different task • Confusion matrix (% same responses): can be interpreted as a dissimilarity matrix Multidimensional scaling Jacobowitz (see Young, 1974): • Children and adults judged the similarity of all pairs of 15 parts of the human body • Task: rank ordering of similarity to a standard dissimilarity matrix Multidimensional scaling 7-year-olds adults Multidimensional scaling • Hair (long/short) • Jaw (smooth/rugged) • Eye (bright/dark) Multidimensional scaling Additional perceptual dimension revealed Multidimensional scaling Multidimensional scaling • Directly asking about the perceptual dimensions: – requires prior knowledge – introduces bias • MDS: – no prior assumptions about the possible dimensions (exploratory) – no response bias • Reveals the hidden structure of the data • MDS is about relationships among stimuli (does not tell us about the perception of individual entities)