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PONTIFICIAE ACADEMIAE SCIENTIARVM SCRIPTA VARIA -

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to know what would be the degrees of freedom, that is, in the sense
of independent observations, which might be available in concrete
examples,

FISHER

Professor MAHALANOBIS wants to know if it is possible to break
up the data into two random partitions. Presumably, one would
want to do this to avoid the double use of data which I have just
suggested is not present. I take it that what one would do would
be to use one set of data to choose the instruments and the other
set of data to estimate the parameters.
As indicated, I do not believe that this is necessary. In addition,
it would be impossible in practice. The Brookings-SSRC model in
its first form has something like one hundred equations; in its second,
 more disaggregate form, there will be many, many more.
Each equation involves at least a few parameters. The data will
run from the war until about 1962 by quarters, so that rough calculation
 gives about 64 observations. There are, however, an enormous
 number of eligible candidates for instruments, far greater
than 64. I do not believe that one would want to use only 32 observations
 either for the estimation of the parameters, or for the choice
of instruments from a list which will be far, far longer than 32.
However, as stated, since I believe the answer to Professor Koop-MANS’
 question to be in the negative, the issue need not arise.

16] Fisher - pag. 80