Weight Discrimination psychology experiment

                                                 WEIGHT DISCRIMINATION

Introduction: The method of constant stimuli means that threshold is determined by presenting the observer with a set of stimuli of which some are above the threshold and of which some are below the threshold but that the set of stimuli are presented in a random order.

Name of the experiment: Weight Discrimination

Purpose:   To determine differential threshold (DL) of the weights with the help of constant stimuli

Details of the subject:

  • Gender ……………..
  • Age………………………………,
  • Qualification…………………………………………

Material used: Weight Box, blind fold, random table, metronome, paper and pen ( weight discrimination  )                                                                        Weight Discrimination


Preliminary setup: First of all the random table for presentation of the stimuli for the ten trials was
made ensuring that each weight has equal probability of being selected and also that same weights don’t occur on the same serial position. This is done for 10 trials. Two observation tables were prepared, one for randomised presentation of variable stimulus (Sv) to be paired with standard stimulus (Ss) for ten
trials and another for noting the responses of the subject.


Procedure: The subject was called to the lab and seated comfortably. A rapport was established during which her details were sought. Thereafter she was instructed as follows- “I will be putting a blindfold on your eyes. Thereafter I will be putting a weight on your palm which would be followed by another weight. You have to tell me whether the second weight is heavier, equal or lighter than the first weight. Be very careful in giving your judgements.”


The subject was blindfolded and was given the weight for two second with the help of the metronome with the gap of two seconds between the presentation of the Ss and Sv. For first five trials the standard stimulus (Ss) is presented first followed by the variable stimulus (Sv) and for the last five trials the Sv is presented first and Ss follows. For the responses heavier a sign of plus (+); for lighter a sign of minus (-); and for equal a sign of  equal(=) was written in the observation table.  Responses thus obtained were put in the observation table which was prepared serially for the twelve weights. Thereafter the frequency table and the proportion table for heavier, equal and lighter responses were prepared. After this, the upper limen, lower limen , DL, IU AND PSE were calculated.

L  =  Sl+
(Sh – Sl) (.50 –

                       Ph  –  Pl

For upper limen use the data
from ‘heavier proportions’ for lower limen proportion               of ‘lighter’ responses.

IU=upper limen-lower

DL =upper limen-lower

PSE=upper limen +
lower limen/2,



The sequence of
presentation of variable stimulus was randomised to avoid serial order effect.

Care was taken that
all of the weights were paired with standard stimulus in each trial. No weights
were left out.

Each weight was kept
on palm for two seconds only with the help of the tick of metronome.

For first five trials
the standard stimulus (Ss) was presented first followed by  the variable stimulus (Sv)  and for the last five trials the  Sv was presented first followed by Ss .

Weights were lifted
with the help of thumb and first finger and were kept in the centre of  palm of the subject with precaution.


  • Results of all calculations for weight discrimination
  • have to be showed in tables for weight discrimination
  • Presentations of heavier for weight discrimination
  • lighter and equal responses along with DL have to be represented in the graph.

Points to ponder:

Concept of threshold /
limen which can found in any sense modality.

Absolute and
Differential threshold.

Essentials of Methods
of Constant Stimuli

Two stimuli are paired; one
is a standard stimulus (Ss) selected from near the centre of the range and
second is one of the several variable stimuli (Sv).

Presentation of stimuli is

Three types of judgements may
be result, “greater”, “lesser” and “equal” / “doubtful”/ “undecided”.

Interval of uncertainty (IU),
Point of Subjective Equality (PSE), Weber ratio.

Evaluation and applications
of method of constant stimuli.

Other methods of
calculating DL

Practice setting the metronome
so that there is a gap of .5 sec., 1.2 sec.,1.5 sec., 2.0 sec. etc. between the
two ticks.

Utility of
experimental controls

Weber ratio= DL/Ss


Guilford, J.P. (1954) – Psychometric
Methods, Tata McGraw-Hill Publishing.

Woodworth, R.S. & Schlosberg, H.(1938).
Experimental Psychology, Oxford & I B H Publishing 


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