AutoFill with Random Numbers

Excel includes a feature that allows you to automatically fill a range of cells with information you have placed in just a few cells. For instance, you could enter the value 1 in a cell, and then 2 in the cell just beneath it. If you then select the two cells and drag the small black handle at the bottom right corner of the selection, you can fill any number of cells with incrementing numbers. This AutoFill feature sure beats having to type in all the values!

You may wonder if there is a similar way to use the AutoFill feature to place random numbers in a range. Unfortunately, the AutoFill feature was never meant for random numbers. Why? Because AutoFill uses predictive calculations to determine what to enter into a range of cells. For example, if you entered 1 into one cell and 5 into the next, highlighted the cells and then used AutoFill, the next number entered in the cell below would be 9 because Excel can deduce that the increment is 4. It is a constant increment that can be predicted.

Random numbers on the other hand are, well, random. By nature they cannot be predicted, else they wouldn't be random. Therefore the predictive nature of AutoFill cannot be applied to random numbers.

However, there are ways around this. One is to simply use the various formulas (using RAND and RANDBETWEEN) that have already been adequately covered in other issues of ExcelTips. These formulas can quickly and easily be copied over a range of cells, using a variety of copying techniques.

Another approach is to use a feature of the Analysis ToolPak which makes putting random numbers into a range of cells pretty easy. Just follow these steps:

1. Choose Data Analysis from the Tools menu. (If you don't see the Data Analysis option on the Tools menu, it means that you don't have the Analysis ToolPak enabled.) Excel displays the Data Analysis dialog box. (See Figure 1.)



Figure 1. The Data Analysis dialog box.

2. In the list of functions in the dialog box, choose Random Number Generation.
3. Click on OK. Excel displays the Random Number Generation dialog box. (See Figure 2.)



Figure 2. The Random Number Generation dialog box.

4. Using the controls in the dialog box, indicate the parameters you want used in generating a range of random numbers. (Make sure that you specify a range of cells in the Output Options area of the dialog box.)
5. Click on OK.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (2964) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: AutoFill with Random Numbers.