When an IF test uses real numbers, problems with round off can cause
incorrect evaluations. For example, say you have the formula
IF(A1+A2=A3,"OK","NOT OK")
and cell A1 contains 2.1, A2 contains 1.2, and A3 contains A1+A2.
Even though the values look identical, the formula will return "NOT OK",
because values in an IF statement (A1+A2) have a different precision than
values in a cell (A3).
So, when the two values are compared, they are not the same ("NOT OK"),
even though logically they are. To avoid this situation, use integer
numbers for equality tests -- or, when you use real numbers, add an
INT function to your IF statement.