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Stock-based compensation Fair value assumptions (Details)	9 Months Ended
	Oct. 31, 2012	Oct. 31, 2011
Minimum [Member]
Fair value assumptions [Line Items]
Expected volatility	58.12%	51.72%
Risk free interest rate	0.74%	1.54%
Dividend yield	0.00%	0.00%
Expected life	4 years 10 months 24 days	4 years 10 months 24 days
Maximum [Member]
Fair value assumptions [Line Items]
Expected volatility	66.82%	66.82%
Risk free interest rate	2.82%	5.13%
Dividend yield	0.00%	0.00%
Expected life	5 years 8 months 12 days	5 years 8 months 12 days

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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