Calibration of probability assessments by professional blackjack dealers, statistical experts, and lay people

The question addressed in this study is whether experts are better calibrated than lay people. We investigated how well people are calibrated when they assess the probabilities of card combinations in the game of blackjack. Three groups of subjects were used: professional dealers, statistical experts, and control subjects. The results showed that experience and statistical expertise do not make people better calibrated in this task. It is argued that the concept of calibration is not wholly appropriate to describe the obtained deviations from the normatively correct responses. This is illustrated by a discriminant analysis performed on the signed deviation scores, which resulted in an almost perfect separation of the three groups, although they were overlapping with respect to calibration.     

There is no induced motion at near-threshold velocities

Duncker's classic experiments on induced motion at near-threshold velocities were based on the assumption that movement of one stimulus relative to the other (object-relative) is perfectly detected while it remains completely impossible to identify which of the two stimuli is moving (subject-relative). In the present experiments it is shown that the threshold areas of object-relative and subject-relative movement are largely overlapping. Consequently Dunker's assumption cannot be fulfilled: whenever object-relative movement is well detectable accuracy of detecting subject-relative motion will also be over 50%. In four additional experiments it is shown that the well-established effects of fixation, stimulus size and enclosure on induced motion are to be interpreted as effects of these variables on the detection of subject-relative movement: the so-called induction phenomena occured only when accuracy was over 50%.     

Recovery from aphasia: Spontaneous speech versus language comprehension

Seventy-four aphasic patients, subdivided into four groups (fluent, mixed, nonfluent, and severely nonfluent), were tested three times in the course of 1 year to assess recovery of spontaneous speech and sentence comprehension. Although 12 of the 28 spontaneous speech variables employed showed significant time changes, some of these changes indicated improvement and others deterioration. There was no overall clinical improvement in spontaneous speech in any group. On the sentence comprehension tests, however, all four groups did show considerable, significant improvement. There were no qualitative or quantitative differences among the groups in the course of recovery, despite the fact that the groups differed in the severity of aphasia as well as on the fluency dimension. In certain patients there was also some improvement in spontaneous speech, but this did not in most cases correlate with an improvement in either fluency or sentence comprehension. Possible reasons why receptive abilities improved more than expressive abilities are discussed.     

Inverse statistics and misperception of exponential growth

Exponential growth presented by numerical series or graphs is grossly underestimated by human subjects. This misperception was considerably lessened by presenting decreasing functions; this conclusion holds for both numeric and graphic stimuli. In the numerical conditions, about 25% of the subjects performed according to the statistical norm. In contrast with previous results, considerable individual differences with respect to sensitivity for rate of growth were observed. This finding was interpreted in terms of task difficulty: Extrapolation of ascending series is too difficult a task to be discriminative. Extrapolation of descending series is much easier, and may therefore better discriminate among subjects.     

Adolescents’ perceptions of health and well-being: Influences of urban contemporary music

The study aimed to explore adolescents’ perceptions of how urban contemporary music influences health and well-being among them. Data on health and well-being effects of music consumption were gathered from a convenience sample of 16 participants (male = 50%, females = 50%) between the ages of 15 and 17 in a series of two focus group interviews. The data were analysed using thematic analysis. Findings suggest the adolescents to perceive both positive and negative influences of contemporary music on their health and well-being. Positive influences included emotional well-being and sense of social belongingness. Negative influences included promotion of risky behaviours, including substance use, misogyny, sexual behaviour and violence. The findings suggest that urban contemporary music serves as a tool of normalisation as well as a catalyst for encouraging risky behaviour in adolescence.     

Stevens vs Fechner: A plea for dismissal of the case

Fechner's Law is based on the assumption that jnd's can be concatenated in order to get a measure of sensation. Stevens contended this assumption; he argued that measurement of sensation by measuring jnd's is ‘indirect’. His ‘direct’ methods, however, appear to be much less direct than originally assumed. Especially Stevens' (implicit) assumption that subjects use numbers on an absolute scale appears to be unwarranted. Ratio scaling is therefore nothing more than cross-modality matching between the number scale and another modality. This view has two consequences. Firstly, it is quite natural to find power functions by cross-modality matching, even when Fechner's Law holds for both modalities. Secondly, if Stevens' power law is true, there will be an overestimation of all exponents by a factor 2.5. All typical power functions will be concave downward after correction for this effect. The discussion boils down to a discrimination between two quite similar curve families: log functions and power functions with an exponent below 0.6. The practical significance of this discussion is doubted.     

Misperception of exponential growth

Exponential growth in numerical series and graphs is grossly underestimated in an intuitive extrapolation task. Subjects’ extrapolations are well described by a model with two parameters only: one for underestimation of the nonlinear growth, the other for linear compensation. The size of the effect is considerable; it is not unusual that two-thirds of the subjects produce estimates below 10% of the normative value. The effect increases with the exponent of the stimulus series, and with addition of a constant to the stimulus series. Neither special instructions about the nature of exponential growth nor daily experience with growth processes enhanced the extrapolations.