Reader’s Digest published this quotation in 1937:
 
“He uses statistics as a drunken man uses lampposts — for support rather than for illumination (Andrew Lang).”
 
It has long been assumed that the quotation’s author is Andrew Lang (1844-1912), a Scots man of letters. However, the quotation has not been found in Lang’s work and was attributed to him twenty-five years after his death. It’s possible that another person—also with the name of “Andrew Lang”—authored the quotation.
 
“Politicians” is an addition to the saying from at least 1963: “Some politicians use statistics as a drunken man uses lampposts—for support rather than for illumination.”
   
 
Wikiquote: Andrew Lang
Andrew Lang (March 31, 1844, Selkirk – July 20, 1912, Banchory, Kincardineshire) was a prolific Scots man of letters. He was a poet, novelist, and literary critic, and contributor to anthropology. He now is best known as the collector of folk and fairy tales, including the works of Brothers Grimm, Charles Perrault and Hans Christian Andersen.
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Attributed
He uses statistics as a drunken man uses lamp posts — for support rather than illumination.
. Widely attributed to Lang (e.g. in Elizabeth M. Knowles, The Oxford Dictionary of Quotations, Oxford University Press; and in Robert Andrews, The Columbia Dictionary of Quotations, Columbia University Press) but the original source has not been found.
 
Googe Books
Reader’s Digest
Volume 30
1937
Pg. 60:
He uses statistics as a drunken man uses lampposts — for support rather than for illumination (Andrew Lang).
 
Google Books
Essentials of Debate
By John Reinder Pelsma
New York, NY: Thomas Y. Crowell Co.
1937
Pg. 72:
“Some people,” said Andrew Lang, “use statistics as a drunk man uses lamp-posts — for support rather than for illumination.”
 
Google Books
Improving Instruction;
Supervision by principals of secondary schools
By Thomas Henry Briggs
New York, NY: Macmillan
1939
Pg. 551:
Statistics are for the purpose of helping one to ascertain the truth, and not for the purpose of justifying a presupposition. There is a type of worker who, in the words of Andrew Lang, “uses statistics as a drunken man uses lampposts—for support rather than illumination.”
 
Google Books
Speaker’s Handbook of Epigrams and Witticisms
By Herbert V. Prochnow
Blackpool: A. Thomas & Co.
1955
Pg. 287:
He uses statistics as a drunken man uses lampposts — for support rather than for illumination.  Andrew Lang
 
Google Books
15 October 1963, The Dispatch (Lexington, NC), “Try and Stop Me” by Bennett Cerf, pg. 4, col. 3:
QUOTABLE:
“Some politicians use statistics as a drunken man uses lampposts—for support rather than for illumination.”—Andrew Lang.
   
Google Books
Social Economics
By C. T. Sandford
London: Heinemann
1977
Pg. 15:
It has been said by one of the more honest politicians that ‘Politicians use statistics as a drunken man uses a lamp-post — for support and not illumination’, and this failing is not confined to politicians.
 
Google Books
New York Murder Mystery:
The true story behind the crime crash of the 1990s
By Andrew Karmen
New York, NY: New York University Press
2000
Pg. 18:
An old adage warns that politicians use statistics like drunks use lamp posts: for support, not illumination.
 
The Economist
Summing it up
No party gets full marks for its use of figures
May 4th 2010
It was the Scots man of letters Andrew Lang who coined the perfect phrase to describe the way many politicians use statistics: “as a drunken man uses lamp-posts—for support rather than illumination.”
 
Tallahassee (FL) Democrat
Our Opinion: Making sense
It’s good when reason and statistics meet
6:21 PM, Oct. 21, 2011
The old saying, renewed in practice almost daily here in the capital city, is that politicians use statistics the way drunks use a light post: for support, not illumination.