Via The Alpha and Omega:
Along with correction methods, Peter Kennedy provides a list of ways in which a wrong coefficient sign can occur in regression analysis.
via Journal of Economic Education (Volume 36, Number 1, Winter 2005)
must read: Peter Kennedy, Sinning in the Basement: What are the Rules? Ten Commandments of Applied Econometrics (free), Applied Econometrics, 2002: 569-589
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