Z-Score Strategy

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//  Copyright by HPotter v1.0 07/07/2014
// The author of this indicator is Veronique Valcu. The z-score (z) for a data 
// item x measures the distance (in standard deviations StdDev) and direction 
// of the item from its mean (U):
//     z = (x-StdDev) / U
// A value of zero indicates that the data item x is equal to the mean U, while 
// positive or negative values show that the data item is above (x>U) or below 
// (x Values of +2 and -2 show that the data item is two standard deviations 
// above or below the chosen mean, respectively, and over 95.5% of all data 
// items are contained within these two horizontal references (see Figure 1).
// We substitute x with the closing price C, the mean U with simple moving 
// average (SMA) of n periods (n), and StdDev with the standard deviation of 
// closing prices for n periods, the above formula becomes:
//     Z_score = (C - SMA(n)) / StdDev(C,n)
// The z-score indicator is not new, but its use can be seen as a supplement to 
// Bollinger bands. It offers a simple way to assess the position of the price 
// vis-a-vis its resistance and support levels expressed by the Bollinger Bands. 
// In addition, crossings of z-score averages may signal the start or the end of 
// a tradable trend. Traders may take a step further and look for stronger signals 
// by identifying common crossing points of z-score, its average, and average of average. 
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study(title="Z-Score Strategy", shorttitle="Z-Score Strategy")
Period = input(20, minval=1)
Trigger = input(0)
hline(Trigger, color=purple, linestyle=line)
xStdDev = stdev(close, Period)
xMA = sma(close, Period)
nRes = (close - xMA) / xStdDev
pos =	iff(nRes > Trigger, 1,
	    iff(nRes < Trigger, -1, nz(pos[1], 0))) 
barcolor(pos == -1 ? red: pos == 1 ? green : blue )
plot(nRes, color=blue, title="Z-Score")