Note 4 - Stock-based Compensation - Assumptions (Details) - Share-based Payment Arrangement, Option [Member] - $ / shares	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020
Dividend yield	0.00%	0.00%
Expected volatility	51.60%	40.30%
Expected life of option (years) (Year)	10 years	10 years
Risk-free interest rate	0.50%	1.40%
Fair value of each option on grant date (in dollars per share)	$ 6.84	$ 2.90

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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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.

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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

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 2
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