I have seen different average volume on different web sites for the same stock. For example INVESTORS.COM (Investors Business Daily) has an average volume different from Quicken.com for the same company. Can you explain why?

The volume we find the most helpful and the volume we use for comparison is the average of volume over the past 50 trading days. Most charting programs allow you to set the days you want to use for average volume. Investor’s Business Daily uses a 50 day volume average. Quicken.com states that it uses a 10-day volume average for stocks in its Quotes area. That appears to be the difference.