An "arithmetic if" statement was the IF statement of very early Fortran. The conventional IF statement (aka "logical" if) was introduced in Fortran IV in 1962 (http://en.wikipedia.org/wiki/Fortran#FORTRAN_IV). An arithmetic IF statement is the sign of really old code (FORTRAN II !), or a programmer following such a style.
The Arithmetic If statement was listed as "obsolescent" in Fortran 90 as a warning that the feature might be deleted from the language in the future.
