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Psycho physical methods 
The branch of psychology which is concerned with the study of rela-
tionship between physical and psychological phenomenon is called psycho-
physics. In this sense, the term psycho-physical methods may be referred to 
all those methods in which attempts are made to employ physical devices for 
the scientific measurement of some psychological experiences like sensation 
of weight, brightness, loudness and other such magnitudes. The other compli-
cated psychological phenomena like sleep or span of memory etc., can also be 
studied through the use of such psycho-physical methods. 
Three classical psycho-physical methods devised by the German physi-
ologist and physicist Gustav Fechner (1801-1887), the father of psycho-
physics are still in vogue. They are: 
(i) 
The method of minimal changes or methods of limits. 
(ii) 
The method of constant stimuli or the method of right and 
wrong responses. 
(iii) The method of average or mean error. 

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These methods are primarily employed to measure absolute threshold 
and the difference threshold. Both the absolute threshold and the difference 
threshold are statistical concepts and both are measured in much the same 
way. 
 
The absolute threshold may be defined as the minimum value of a phys-
ical stimulus that reliably produces sensation. Absolute threshold, thus, sepa-
rates the sounds we can hear from those we cannot, the odors we can smell 
from those we cannot, the brightness of the light we can see that which cannot 
and so on. 
 
The difference threshold may be defined as the minimum difference in 
value between two stimuli that can be perceived by the subject. For example, 
when one experiences a particular weight put on his hand, the difference 
threshold is the minimum weight that should be added to it so that the total 
becomes distinguishable from the first. Let us now give a brief idea of the 
three psycho-physical methods mentioned above.