Beta: Indicator Documentation
Syntax
public Beta(Bars bars, Bars index, int period)
Parameter Description
  
    | bars | Primary Bars object | 
  
    | index | The Bars object for your index symbol | 
  
    | period | Lookback period | 
Description
According to definition at 
Investopedia, beta coefficient is a measure of the volatility (risk) of a security or a portfolio in comparison to the market as a whole. 
Here we are using ^GSPC symbol of the S&P500 Index, suitable for Yahoo! data.
Example
using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using WealthLab;
using WealthLab.Indicators;
using Community.Indicators;
namespace WealthLab.Strategies
{
	public class MyStrategy : WealthScript
	{
		protected override void Execute()
		{
			Beta beta = Beta.Series( Bars, GetExternalSymbol("^GSPC",true),200);  // Yahoo symbol
			ChartPane betatest = CreatePane( 30, true, true );
			PlotSeries( betatest, beta, Color.DarkMagenta, LineStyle.Solid, 2 );
		}
	}
}