I was reading this Investopedia article How To Use Volume To Improve Your Trading, which talks about high and low volumes.
Does "high volume" mean high compared to itself in the past? Or compared to other stocks? or what? Is there a generally accepted number across the stock market that is the threshold to be considered high?
The daily Volume is usually compared to the average daily volume over the past 50 days for a stock.
High volume is usually considered to be 2 or more times the average daily volume over the last 50 days for that stock, however some traders might set the crireia to be 3x or 4x the ADV for confirmation of a particular pattern or event.
The volume is compared to the ADV of the stock itself, as comparing it to the volume of other stocks would be like comparing apples with oranges, as difference companies would have different number of total stocks available, different levels of liquidity and different levels of volatility, which can all contribute to the volumes traded each day.
Volume is really only valuable when compared to some other volume, either from a historical value, or from some other stock.
The article you linked to doesn't provide specific numbers for you to evaluate whether volume is high or low. Many people simply look at the charts and use a gut feel for whether a day's volume is "high" or "low" in their estimation. Typically, if a day's volume is not significantly taller than the usual volume, you wouldn't call it high. The same goes for low volume.
If you want a more quantitative approach, a simple approach would be to use the normal distribution statistics: Calculate the mean volume and the standard deviation. Anything outside of 1.5 to 2.0 standard deviations (either high or low) could be significant in your analysis. You'll need to pick your own numbers (1.5 or 2.0 are just numbers I pulled out of thin air.)
It's hard to read anything specific into volume, since for every seller, there's a buyer, and each has their reasons for doing so. The article you link to has some good examples of using volume as a basis for strengthening conclusions drawn using other factors.
