// Downloaded From https://www.WiseStockTrader.com
function Beta( BaseSymbol, N )
{
 Chg = ROC( C, 1 );
 ChgBase = ROC( Foreign( BaseSymbol, "C" ), 1 );

 return ( N * Sum( Chg * ChgBase , N ) - Sum( Chg, N) * Sum( ChgBase, N ) ) /
        ( N * Sum( ChgBase ^ 2 , N ) -  Sum( ChgBase, N ) ^ 2 );
}

symbol = ParamStr( "Symbol", "^NDX" );

Plot( Beta( symbol, Param("Periods", 21, 2, 100 ) ), _DEFAULT_NAME(), ParamColor( "Color", ColorCycle ) );