Spreadsheet view - rounding differences explained
There are two common ways to round numbers and they only differ in how they handle numbers that end in a 5 with only zeros after that digit. The first is called "Asymmetric Arithmetic Rounding" or "Round-Half-Up (Asymmetric Implementation)", and is what you commonly see in applications such as Microsoft Excel or Powerpoint. This way always rounds up. So 6.25 rounds to 6.3 and 6.35 rounds to 6.4.
The second is called "unbiased rounding" and "statistician's rounding" among other names. It rounds to the nearest even digit before the rounding position, so 6.25 rounds to 6.2 and 6.35 rounds to 6.4.
If you always round up, the more errors you will accumulate the more you round and you will introduce a bias. In reality, 6.25 is just as close to 6.2 as it is to 6.3, and 6.35 is just as close to 6.3 as it is to 6.4. Using the "unbiased rounding" method produces results that are closer to the actual values (thus the "unbiased" and "statistician's" names for it).
mTAB is designed for those who wish to dive deep in to their data, analyzing % distributions and looking for statistically significant results. Because of this, when mTAB was developed, we ensured that we were using the most appropriate rounding method, which was the "unbiased rounding" method mentioned above.
When comparing the rounding of the same value between mTAB and Excel, you may see slight differences when the circumstance above occurs. Both methods are correct, but one is just considered more accurate.
For more information on rounding methods please refer to: http://en.wikipedia.org/wiki/Rounding