Z-Score Indicator for Amibroker (AFL)

/*-----------------------------------------------------------
MTR Investors Group - www.MTRIG.com

Statistical Z-Score Indicator: Tracks how many standard 
deviations the price is from the mean.

For more information on this indicator see the links below.

MTRIG - Reference
http://blog.mtrig.com/mtrig/blog/page/Z-Score-Indicator.aspx

Z-Score Trader's Notebook Article
http://premium.working-money.com/wm/display.asp?art=344

AMIBroker forumula on Traders.com
http://www.traders.com/Documentation/FEEDbk_docs/Archive/022003/TradersTips/TradersTips.html

Z-Score in Wikipedia
http://en.wikipedia.org/wiki/Standard_score
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periods = Param( "Period", 20, 1,3, 1);

ZScore = ( Close - MA( Close, periods ) ) / StDev( Close, periods );

Plot(ZScore, "Z-Score", colorDarkGrey );
Plot(2, "2", colorBlueGrey);
Plot(0, "0", colorBlueGrey );
Plot(-2, "-2", colorBlueGrey);