1. “Boy Was My Face Red” 4 tips to avoid common spreadsheet errors Jake Carney 4/5/2010 DFMW, Section B

2. Everyone makes spreadsheet errors – even experts! Studies show that over 60% of spreadsheets contain errors Studies show that overconfidence is extremely common among spreadsheet developers The majority of developers who are highly confident that their models are without errors are mistaken 2

3. Tip #1 – Design your model before building it Spreadsheet developers typically begin building models without a preliminary design The design can be as simple as a few notes sketched out on a piece of scrap paper For highly complex models, you can use formal planning techniques including CASE-like tools Quickly estimate the output(s) of your model using rounded numbers and simple math 3

4. Tip #2 – Test your model Most spreadsheet developers skip testing Studies suggest that 80% of spreadsheet errors can be detected through testing Testing should not be conducted by the spreadsheet developer Group testing is proven to be more effective than individual testing; i.e. make sure that 2 or more colleagues test your model 4

5. Tip #3 – Be careful when working with an old model built by someone else Error rates increase when: Models are reused and reworked by someone other than the author Models are modified outside of the originating organization Multiple authors collaborate on the development of a single model 5

6. Tip #4 – Be aware of the most common types of errors Logic Error:a formula is used incorrectly, leading to an incorrect result Reference Error: a formula contains one or more incorrect references to other cells Placing Numbers in a Formula:one or more numbers appear in formulas Copy/Paste Error:a formula is wrong due to inaccurate use of copy/paste Data Input Error:an incorrect data input is used Omission Error: a formula is wrong because one or more of its input cells is blank 6