In his excellent book A Question of Intelligence: The IQ Debate in America, former Fortune magazine editor Daniel Seligman describes what it’s like to take the WAIS-R IQ test.  In part 4 we learned he did poorly on the Picture Completion subtest, and on page 4, he describes his performance on the next subtest:

I did much better on the next exercise: the Digit Span.  This is a test of short-term memory in which the examiner reads off a string of digits and asks you to repeat them.  He begins with three digits and proceeds incrementally to nine.  If you miss twice at any level, you are judged to have scored at the next lower level.  I made it to nine with no special difficulty.

But there is a fiendish round two to Digit Span.  Beginning with two digits and proceeding on to eight, the examiner asks you to repeat the digits backward.  This was a lot tougher.  I found that once we were up to six and seven digits, it helped me to “chunk” them into groups of three or four.  Of course, you still have to hold the first chunk in your head while focusing on the second, and twice during the exercise I lost the first chunk.  On the other hand, I batted a solid .500 on the eight-digit span backward-accomplishing it once in two attempts-and found my morale somewhat restored.

From Seligman’s description, it’s clear that he made it up all the way to the hardest item on both rounds, but got only partial credit twice.  This implies a raw score of 26/28.  This equated to a scaled score of 17 in the peak age group, but in his own age group (55-64) it equated to an astonishing scaled score of 19 (IQ 145, U.S. norms; IQ 144 U.S. white norms).

[Update (Nob 20/2015): because WAIS-R norms were a decade old when Seligman was tested, and the Flynn effect increased WAIS Digit Span scaled scores by 0.059 points a decade from 1978 to 1995 (Flynn, 2012), his scaled scores must be reduced to 16.94 and 18.94 respectively.  An age adjusted scaled score of 18.94 remains equivalent to a spectacular IQ of 145 (U.S. norms; 144 U.S. white norms) on that subtest].

But the great David Wechsler would not have been impressed, writing that “as a test of general intelligence”, Digit Span “is among the poorest.”

In The Measurement of Adult Intelligence (third edition, 1944) Wechsler wrote (pg 83-84):

The ability involved contains little of ‘g’, and as Spearman has shown is more or less independent of the general factor.  Our own results confirm these observations.  For a long time we considered the desirability of eliminating the test from our battery altogether, but finally decided to retain it for the following reasons: (1) While memory span for digits backwards and forwards is on the whole a poor measure of intelligence, it is nevertheless an extremely good one at the lower levels.  Except in cases of special defects or organic disease, adults who cannot retain 5 digits forward and 3 backwards will be found, in nine cases out of ten, to be feebleminded.  (2) Special difficulty with the repetition of digits forwards or backwards is often of diagnostic significance.  Obvious examples are the memory defects which constitute clinical symptoms in certain organic and other types of cases.  A marked falling off in memory span is often one of the earliest indications of them.

According to scholar Arthur Jensen, Digit Span forward has a g loading of 0.3 while Digit Span backward has a g loading of 0.6.   The school board I attended as a kid labeled Digit Span a measure of “Span of Attention”.

[Update: Nov 20/ 2015: Sadly, it has been brought to my attention by commenter “jeanbedelbokassa” that Seligman is actually saying he got 25/28, not 26 out of 28.  This means his scaled score (relative to the peak age group) is 16, and his scaled score relative to his own age is 18.  Because of old norms, these get reduced slightly to 15.94 and 17.94 respectively.  An age adjusted scaled score of 17.94 equals an IQ of 140 (U.S. norms; 139 U.S. whites norms).  Still an incredibly high score.  For making this error, points also need to be deducted from my reading comprehension IQ :-)]