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Stock option plan - Fair value of options granted (Detail)	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Stock option plan
Expected term (years)	6 years 1 month 6 days	6 years 2 months 12 days	6 years 4 months 24 days
Expected volatility	64.40%	62.10%	60.20%
Risk-free interest rate	1.00%	1.30%	2.70%

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- References

No definition available.

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- Definition

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.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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