What time frame do you use in your charts to decide entry/exit for day trading?

I have been pretty confused over this...I normally look at ema crossovers to day trade...But what time frame is a sure shot indicator of an uptrend or a downtrend?

It might show a crossover in 5 minute chart but in the 30 minute chart...It still shows no crossovers. Can anyone here help please? Thanks in advance.

There's no such thing as sure shot, its all a game of probabilities isn't it?

Generally people tend to look at a higher time frame for direction and trade the smaller one.

Thanks a lot for the reply...In pride's intra day tactic he suggests to exit when the scholastic shows a bearish pattern and crosses the line from above in a 5 minute chart. But I have observed that in such a scenario it goes back up pretty fast within the next 10 mins again. So this is whats confusing me.