“An economist is an expert who will know tomorrow why the things he predicted yesterday didn’t happen today” is a jocular one-line saying that has been credited to Kirk Kirkpatrick (1973), Laurence J. Peter (1977) and James R. Faulkner (1983). “An economic expert is a guy who knows tomorrow why the things he said yesterday didn’t happen today” has been cited in print since at least 1949.
   
“A columnist is a man who tells you tomorrow why the things he predicted yesterday didn’t happen today” was printed in a 1950 newspaper column; Evan Esar’s Esar’s Comic Dictionary (1951) also defined “columnist” as “One who knows tomorrow why the things he predicted yesterday didn’t happen today.” However, no other known citation replaces “economist” with “columnist.”
 
     
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Hearst’s International combined with Cosmopolitan
Volume 127
July-December 1949
Pg. 15:
An economic expert is a guy who knows tomorrow why the things he said yesterday didn’t happen today.
Ullie Crawford, “Headline Hopping” The Philadelphia Inquirer, PHILADELPHIA, PENNSYLVANIA
 
8 March 1950, Amarillo (TX) Daily News, “From A to Izzard” by Wez Izzard, pg. 1, col. 1:
An economist has been defined as a man who knows tomorrow why the things he said yesterday didn’t happen today.
 
18 May 1950, Ogden (UT) Standard-Examiner,  ‘Sunshine and Shadow” by Sol, pg. 12A, col. 4:
Hi, ho, weathero—a columnist is a man who tells you tomorrow why the things he predicted yesterday didn’t happen today.
 
Hathi Trust Digital Library
Esar’s Comic Dictionary
Completely Revised and Enlarged Edition
By Evan Esar
New York, NY: Horizon Press
1951
Pg. 66:
columnist
(...)
5. One who knows tomorrow why the things he predicted yesterday didn’t happen today.
 
23 February 1958, Cleveland (OH) Plain Dealer, “Broadway here they come!” by Earl Wilson, pg. 4H, col. 3:
Wilson Sez
An expert in economics is a man who knows tomorrow why the things he predicted yesterday didn’t happen today.
   
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The English Digest
Volumes 72-73
1963
Pg. 62:
EXPERT: Someone who will know tomorrow why the things he predicted yesterday didn’t happen today.
 
11 May 1970, Paris (TX) News,  “Editor’s Notes,” pg. 6, col. 7:
AN ECONOMIST is a fellow who can tell you tomorrow why what he predicted yesterday didn’t happen today.
   
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1974 Ford Almanac
Ford Motor Company
New York, NY: Golden Books
1973
Pg. 67:
Predicting The Future
Expert: A man who will know tomorrow why the things he predicted yesterday didn’t happen today.
 
5 December 1973, Pampa (TX) Daily News,  “It Happened Last Night” by Earl Wilson, pg. 10, col. 6:
REMEMBERED QUOTE: “An economist is a man who’ll figure out tomorrow why the things he predicted yesterday didn’t happen today.”—Kirk Kirkpatrick
   
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The American Legion
Volume 114
1983
Pg. 76:
An economist is an expert who will know tomorrow why the things he predicted yesterday didn’t happen today. —JAMES R. FAULKNER
   
Google Books
20,000 Quips & Quotes
By Evan Esar
New York, NY: Barnes & Noble Books
1995, ©1968
Pg. 255:
An economist is an expert who will know tomorrow why the things he predicted yesterday didn’t happen today.
 
Google Books
QFINANCE: The Ultimate Resource
By Fiona Pike, et al.
London: Bloomsbury Pub.
2012
Pg. ?:
An economist is an expert who will know tomorrow why the things he predicted yesterday didn’t happen today. Laurence J. Peter. Peter ‘s Quotations: Ideas For Our Time (1977)