Quote from elit:
I'm wondering how important a factor slippage is when testing a system that is based on daily or weekly data? (Or points..., FYI I'm using metastock)
I include $20 on each side slippage when testing multi-day strategies with high volume stocks. (assuming 1,000 share lots.) Even so, if a strategy shows only small/fractional gains, then it probably isn't worth pursuing.
Much depends on your entry and exit strategy. If you are buying opens and selling closes on daily bars, your backtesting is likely to be less predictive than if you are using daily data for setups, then watching for intraday entry points. A market order on open or close leaves you open to a lot of variables when you move to real time trading.
I prefer market orders, because I deal with fairly high volume stuff and when I want in, I want in. I will only use limit orders when a spread is wide and threatens to stay that way.
Only actual experience with entries for your strategy will give you the answers you need. But for backtesting, always assume that the real world will not provide perfection. Because it won't.