If I use "lose weight" as one of my keywords, it would include everything that will have the 'lose weight' phase like "how to lose weight", "lose weight fast", "lose weight in one week" and so on. What I want to ask is that if I decide to use only one keyword "lose weight" and leave its other related keywords (like above) and someone searches for something like "lose weight in a week" how much would my rakings suffer because of that keyword (against my competitor who has use the exact match) , provided bids and other things are the same. Would the overall effect (because of the keyword) be marginal or substantial?
The answer is depend on your competitor account overall QS. you try and see the results for your self.
Well I assume you will have [lose weight] in your account, right? So this exact match will be triggered every time someone types for lose weight (providing your Rank is high enough). With regards to things such as lose weight in a week, if you don't have [lose weight in a week] (exact) then it will depend on the QS and Max Bid for the keyword that is triggered. My advice is treat most phrase matches as simply a means of finding out more exact matched keywords or a way of adding negatives. Phrase matches in a very small % of the time can be more efficient than Exact but it's rare. For example if people were searching for red widgets in ##city## and you sold red widgets in every city in the world, it would be sensible to use the phrase keyword "red widgets in" as opposed to entering every single exact match of: [red widgets in manchester] [red widgets in london] etc - unless it was searched a lot in which case it would be wise to add one of the above as exact into your account. The phrase match will pick all the rest up that are rarely searched. I hope that's what you meant?