SQN®

SQN® measures the ratio between the expectancy and the standard deviation of the R-multiple distribution that was realized by a trading system. 

Dr. Tharp has determined that the better the SQN®, the easier it is to use various position sizing strategies to meet one's objectives.

Formula:  SQN® = root(number of trades) * (expectancy/stdev(R-multiple))

SQN® scale for interpretation:

<1.5: Probably Very Hard To Trade.
1.51-2.0: Average System (Should Be At Least 1.7).
2.01 to 3.00: Good System.
3.01 to 5.00: Excellent System.
5.01 to 7.00: Super System (Few Exist).
7.01 Or Higher: Holy Grail System.

Credit: The SQN® (system qualities number) was originally developed by Dr. Van Tharp https://www.vantharp.com/