Generally, when a stock is in its upward trend, its volume follows the price during up-days. On the other hand, when a stock encounters a correction, its volume goes down. This is one of the rules, which defines an uptrend.
Therefore statistically, during an uptrend, volume printed in up-days should be higher than that reported in down-days. If it is not the case, a stock flashes red - something wrong is going on.
Please, look at the chart below:
This chart shows volumes printed by S&P 500, starting from September 2015. Except for October, volume during up-days (when the index closed higher than in the previous trading day) was lower than volume reported in down-days (when the index closed lower than in the previous trading day).
Well, it is not a situation featuring an upward trend. Quite the opposite, it looks like S&P 500 is sending a signal that it is topping or even it is in a correction / bear market mode.
Another chart presents S&P 500 in the medium-term, during its late bull market stage:
As the chart shows, between the beginning of 2013 and the end of 2014 the pattern was supporting a bullish thesis: volume was higher during up-days.
Then, since the beginning of 2015 the pattern has changed - volume has been higher during down-days (with a climax in the third quarter of 2015).
Summarizing - the volume pattern is not supporting a bullish case any more.