A great way to gain diversification without overdiversifying is to use more than one screen in your trading. And to go one step further is to use multiple screens or a "screen of screens" approach to select only stocks that clear more than 1 or 2, or 3 or however many of those individual screens you are using.

This is called Consensus Screening and you can build and backtest Consensus Screens with the Advanced Backtester.

Here is a link to an article which describes the easy process of creating this powerful screening method and backtesting it.