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I was also a little puzzled by the post hoc ergo propter hoc tone of his assertion that "Human beings have no trouble keeping seven ideas in their head (hence t

I was also a little puzzled by the post hoc ergo propter hoc tone of his assertion that "Human beings have no trouble keeping seven ideas in their head (hence the seven digit phone number)", if for no other reason than that I have an easier time remembering my social security number than my own phone number. Positing the existence of the decimal system because humans evolved with ten fingers is something I can get behind. The seven-digit phone number, on the other hand, I'd probably attribute to other causes.

nrr 17y ago
In his case, it'd probably classify under the fallacy of cum hoc, ergo propter hoc given the abuse of statistics. The seven-digit phone number in the NANP most definitely did not come about because of this; in fact, most quoted phone numbers from the early days of telephones in the US had five digits. You can see parts of the numbering plan reminiscent of this even today. Case in point, I grew up in a St. Louis suburb called O'Fallon, where the predominant telephone branch exchange was 632. The first two digits correspond to OF as found on the keypad or rotary dial. From there, the extension was five digits. Belleville is very much the same way with the 23x exchanges used there. If you were calling to someplace out of town, you'd name the branch exchange and then enumerate the extension. For example, to call the Belleville municipal police department from O'Fallon, you'd tell the operator that you're trying to dial "Belleville 4-1218."