List of spacewalkers

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In the branch of experimental psychology focused on sense, sensation, and perception, which is called psychophysics, a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time (absolute threshold). This limen is also known as the difference limen, difference threshold, or least perceptible difference.
In the branch of experimental psychology focused on sense, sensation, and perception, which is called psychophysics, a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time (absolute threshold). This limen is also known as the difference limen, difference threshold, or least perceptible difference. For many sensory modalities, over a wide range of stimulus magnitudes sufficiently far from the upper and lower limits of perception, the 'JND' is a fixed proportion of the reference sensory level, and so the ratio of the JND/reference is roughly constant (that is the JND is a constant proportion/percentage of the reference level). Measured in physical units, we have: where is the original intensity of the particular stimulation, is the addition to it required for the change to be perceived (the JND), and k is a constant. This rule was first discovered by Ernst Heinrich Weber (1795–1878), an anatomist and physiologist, in experiments on the thresholds of perception of lifted weights. A theoretical rationale (not universally accepted) was subsequently provided by Gustav Fechner, so the rule is therefore known either as the Weber Law or as the Weber–Fechner law; the constant k is called the Weber constant. It is true, at least to a good approximation, of many but not all sensory dimensions, for example the brightness of lights, and the intensity and the pitch of sounds. It is not true, however, for the wavelength of light. Stanley Smith Stevens argued that it would hold only for what he called prothetic sensory continua, where change of input takes the form of increase in intensity or something obviously analogous; it would not hold for metathetic continua, where change of input produces a qualitative rather than a quantitative change of the percept. Stevens developed his own law, called Stevens' Power Law, that raises the stimulus to a constant power while, like Weber, also multiplying it by a constant factor in order to achieve the perceived stimulus. The JND is a statistical, rather than an exact quantity: from trial to trial, the difference that a given person notices will vary somewhat, and it is therefore necessary to conduct many trials in order to determine the threshold. The JND usually reported is the difference that a person notices on 50% of trials. If a different proportion is used, this should be included in the description—for example one might report the value of the "75% JND". Modern approaches to psychophysics, for example signal detection theory, imply that the observed JND, even in this statistical sense, is not an absolute quantity, but will depend on situational and motivational as well as perceptual factors. For example, when a researcher flashes a very dim light, a participant may report seeing it on some trials but not on others. The JND formula has an objective interpretation (implied at the start of this entry) as the disparity between levels of the presented stimulus that is detected on 50% of occasions by a particular observed response (Torgerson, 1958), rather than what is subjectively "noticed" or as a difference in magnitudes of consciously experienced 'sensations'. This 50%-discriminated disparity can be used as a universal unit of measurement of the psychological distance of the level of a feature in an object or situation and an internal standard of comparison in memory, such as the 'template' for a category or the 'norm' of recognition (Booth & Freeman, 1993). The JND-scaled distances from norm can be combined among observed and inferred psychophysical functions to generate diagnostics among hypothesised information-transforming (mental) processes mediating observed quantitative judgments (Richardson & Booth, 1993).

David Nabarro
David Nabarro CBE (born 26 August 1949) is a Special Envoy on Covid-19 for the World Health Organization. He has made his career in the international civil service, working for either the Secretary-General of the United Nations or the Director-General of the World Health Organization. Since February 2020, he has helped the DGWHO deal with the COVID-19 pandemic.

This is a list of all astronauts who have engaged in an EVA by partly or fully leaving a spacecraft, exclusive of extravehicular activity on the lunar surface. It is ordered chronologically by the date of first spacewalk. For a list of astronauts who have performed lunar EVA ("moonwalks") see List of Apollo astronauts who walked on the Moon. For the 2017 docudrama film, see The Spacewalker.
This is a list of all astronauts who have engaged in an EVA by partly or fully leaving a spacecraft, exclusive of extravehicular activity on the lunar surface. It is ordered chronologically by the date of first spacewalk. For a list of astronauts who have performed lunar EVA ("moonwalks") see List of Apollo astronauts who walked on the Moon. For the 2017 docudrama film, see The Spacewalker. The following 11 countries have flown spacewalkers: United States of America 129, Russia (formerly Soviet Union) 65, Canada, France, Japan 4, Germany 3 , China 2, Italy, Switzerland, Sweden and Great Britain 1 each. There have been 15 women who have taken part in an EVA.

Just-noticeable difference