Outside bets 馃捀 in roulette carry the lowest risk. They cover more numbers yet the payouts are lower (1/1 for Red, Black, Odd, 馃捀 Even, 1 18, 19 36). These outside bets carry the best prospects for long-term wins.
If you want to know what is the probability that the coin will come up heads, then 馃ざ that would be: heads / (heads + tails) = 1/2 = .5. Likewise when playing an even money bet 馃ざ at roulette, that option covers 18 of the 37 possible outcomes: 18/37=. 48648649.
Martingale is definitely one 馃ざ of the most widely known and most successful roulette strategies (for some it is the best roulette strategy), and it 馃ざ is very simple to apply. It can be used only for outside bets, those that provide almost 50% chance of 馃ざ winning, and excluding the 0.