Once there is a mistake in your formula, you will see .Null. values in your column cells. If your formula are not a simple one, you may not be able to easily find out the cause of the problem. In general, when creating a complicated formula, we will follow the following rules.
|•||Rather than building the complete formula in one time, you should write a simple part of it first.|
|•||If the simple part returns correct values, you will then add more to the formula and do the test again.|
|•||This add and test process should be repeat and repeat until a complete formula is formed.|