Does anybody know if i delete all inactive keywords in my account, will it increase my quality score which in turn would decrease my min. cpc and my costs?
no, you can ignore the inactive keywords and left they there, no clean up needed. Also is recommended to left they there as they can came alive if the market change for a specific keyword (very rare) or you decided to make it alive by your self biding some value in the future.
but google in there online manuals have suggested that one should delete those keywords, inactive keywords have a negative result on quality score.
no it doesn't it say a complete different think: if you want to reset the history for that specific keyword you can delete it and add it again - but if you do this procedure for a keyword that is not relevant you will decrease the performance of your complete ad group - but only if you begin to delete and add again a same a not relevant keyword - is the opposite of what you thinks, so do not delete the keyword to preserve your quality score. It recommends that if you do not want to bid or improve the the ad quality you just leave it there for future use or if you have luck it became available again for future use.
The quality score is assigned at the keyword level. One keyword's quality score has no impact on the quality score of other keywords in the account.
I add, edit and delete keywords on a daily basis. What do you mean by "campaign gets reset"? I have had them manually review my site after making changes but campaigns continue on as usual...I thought.
When you make adjustments to the keywords, for example, adding some new ones, your campaign will lose some of it's 'earned ranking' in the ad column placements; e.g. your ad campaign has earned a ranking place of #7 in where it gets displayed. You will usually see it drop slightly. Try it out. Edit your campaign settings but first of all, go to the estimated clicks / placement before you actually edit anything and estimate your traffic / clicks, but take note of the placement of the best keywords. Add some extra keywords and then estimate the traffic again. You will see a dip in some of your placements.
I always thought adding up the quality score of all the keywords would sum up to the quality score of the whole account.
This really clarified a lot for me. I have been searching for hours. My main source of traffic is through adwords. Thanx