Kanzler is correct. Spread is the silent enemy of the retail trader.
Traders moan about slippage, poor fills, re-quotes, etc, but happily accept spreads as a necessary cost of conducting business. Sure, there's nothing overly wrong with that thinking, but ironically, traders invariably forfeit far more money to spreads (and commissions) than they ever do to slippage.
On the "Why do 95% of traders lose?" type threads, the most frequent reasons given are poor methodology, lack of discipline, emotional trading, etc. Yet transaction costs are possibly the biggest single reason that most daytraders lose. If, for example, you're trying to scalp 10 pip moves, a 2 pip spread is a 20% edge against you, before you even start to apply your strategy against the market. Unless you have an extraordinary scalping strategy, you'd get much better odds playing roulette at your local casino.
There are also "why not invert a losing strategy?" threads, and the primary reason why this will not work profitably is....... transaction costs.
Spread reduces the profit on your winning trades, and increases the loss on your losing trades. When you calculate net RR, you must add the spread to the risk AND deduct the spread from your return. It doesn't matter how great your strategy is, or your risk management, or your implementation, the spread will always work against you.
One of the simplest, yet most effective, ways of improving your bottom line, is to find br0kers, and pairs, with the lowest spreads. Or at least be willing to increase your TP on pairs with higher spreads; for example, if you're willing to accept a 2 pip spread when chasing a 50 pip move, then you should be looking to trade 100 pip moves on a pair with a 4 pip spread, to keep the 'house edge' at 4%.
Expressing spread as a % of average daily range is a useful metric. Everything else being equal, trade pairs with the smallest spread/ADR ratio. Both the Signal and DisplayInfo indicators that I wrote display this ratio.