Bauhaus-Universität Weimar

Experimental Psychology: A Manual of Laboratory Practice, Vol. II: Quantitative Experiments, part 1: Student's Manual
Titchener, Edward B.
Tiw Metrie Methods 
experiment ; its results cannot be counted in with the rest. The prevention 
of the error of bias is indefinitely better and easier than its cure. And, since 
E already has his directions with regard to variation of the length of the 
series, its prevention lies in O's hands.—Cf p. 4 above. 
The I series furnishes an 1\ which marks the point at which 
O's judgment changes from ‘ lighter ’ to ‘ same’, ‘ doubtful ’ or 
‘ darker.’ This value of 1\ is recorded as rd (d= descending). The 
I series furnishes an 1\ which marks the point at which O's judg¬ 
ment changes from ‘ same ’ or ‘doubtful ’ to ‘ lighter.’ This value 
of rx is recorded as ra (a=ascending). The difference rd—r is 
recorded as Ard; the difference ra—ras Ara. The average of 
A rd and A ra is written A r. The final A r is the average of all 
the separate Ard and Ara values. If, as was suggested above, 
we take 10 paired series, in each of the two positions of space, the 
final A'" will therefore be the average of 40 determinations. 
We have now, if we have worked aright, a value of the DL 
(this final Ar) which is unaffected by any constant error, and is 
as free from variable and accidental errors as, in the time at our 
disposal, we can make it. The tale of errors is long ; and the 
reader who has followed the preceding discussion may, perhaps, 
be inclined to distrust a method that has to move so warily among 
so many pitfalls. But the sources of error are there : we must 
either avoid them or fall into them : and that is, surely, the best 
rule of work which takes account most fully and explicitly of the 
dangers that beset the path of the experimenter. It is, in reality, 
an advantage of the method of limits that the errors involved 
are obvious, can be separately traced and definitely named : for 
that means that they can, with some little effort, be eliminated. 
And in spite of the number of errors, the course of the method 
in practice is comparatively simple. All that E has to do is to 
grade his R according to rule ; all that 0 has to do is to compare, 
with maximal attention, every pair of R presented. 
In conclusion, the information which the method gives may be 
summed up under the headings : magnitude, course and preci¬ 
sion of the DL, and magnitude and direction of the constant 
(1) Magnitude of the DL.—The absolute magnitude of the


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