Are people making stock trades through brokers like Scottrade any better or worse, on average, than a random computer picking stocks? Is a non-experienced stock picker likely to lose their money, or more likely to stay more or less in line with the general trend of the market?
Worse. Individual traders trade at the wrong time and let emotion get in the way (look at everyone selling now, when prices are down - they would have been better off selling a couple of months ago, when they were high). They end up buying high and selling low. And they rack up fees by trading too often.
While this article is actually a critique of Jim Cramer, it includes:
Yes. Some professional strategists may even exploit this by determining what small investors are doing (e.g. via odd-lot trade statistics) and doing the opposite. Since any trade by an individual is likely to be wrong, Buy-and-Hold is a good approach. If you buy a stock and sell it ten years later, you’ve made at most two poor trades in a decade! 
In OP you speak of stock-pickers (who might hold a stock for a year or more) but title speaks of “trading” which might involve holding a stock for a few days or less. I think the cases need to be separated.
There are trading strategies that have an excellent chance at making a little money, and may deceive a day-trader to think he’s smarter than he is. In another thread an expert trader referred to such strategies as “gathering pennies in front of a bus.” Eventually it leads to a big loss. (A simple example of such a strategy might be the Martingale of casino fame, where you almost always win a dollar, but sometimes lose your entire bankroll.)