Arbitrage trading is a separate class of strategies, where there is no concept of purely market risk. Therefore, an arbitrage is a trade with endless profitability, with a positive profit when the risk to zero.
Developing a strategy for trading on the stock exchange, the trader is constantly looking for a balance between risk and profit. The greater the ratio of profit to risk, the more effective the strategy works — this is the easiest way to calculate the profitability of trade.