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. |

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