Google Adwords lets you create keywords and key phrases against which your advertisements are run. More often than not you will run into a scenario where you will need to delete or update one or more of your keywords.
You should remember that updating or making any kind of changes to a keyword is pretty much equivalent to deleting that keyword and creating it again in most cases. This will cause all the statistics associated with that particular keyword to be reset to zero……ok, well….kind of. Moving the keyword to another group will also cause the same effect. If you like to preserve the statistics then pausing the keyword is a better option than deleting it.
Even though your statistics seems to have been reset to zero, what actually happens is that you have created a brand new keyword with no statistics. Your previous keyword instance (which is different even though the keyword itself is the same) will keep the acquired stats till when it is deleted.
Account history is one of the many factors that contribute to the bidding process and other statistic calculations including the quality score of the keyword. The stats from both the keyword instances, new and old, will therefore affect your account history.
Why you should delete keywords
Keyword maintenance is essential to maintain a good adword account history which will eventually give you a good ROI in the long run. It will result in a good quality score for your keyword which will allow you to rank better and pay less. Remember that the Quality Score is calculated based on many different factors including the account history.
Deleting low performing keywords or keywords with very low CTR will allow you to improve the CTR of the adgroup and the campaign as a whole. This will also improve the CTR of your ads, now that it will only be shown for keywords with a good CTR. Ads with a good CTR will usually be shown more often than the ads with a lower CTR.
If you have a lot of keywords, then it is easier to manage if you delete the poor performing keywords.
Another option is to pause the keywords rather than deleting them. Pausing the keyword has some advantages than deleting them outright. It is usually a better option if you are not sure about the deletion or you plan to re-enable them at a later time.
It saves the past statistics of the keyword and do not reset it. This will also save your quality score for the keyword. So if you have keywords with high quality scores, which you might want to use again at a later time then pausing will be a better option than deleting them.
Moving keywords to other ad groups
Sometimes you might want to move a keyword to a different ad group or even a different campaign. If you have a high performing set of related keywords, it is a good candidate to be moved to new ad group so that you can manage the budget better.
Sometimes you might have a low performing keyword just because the corresponding ad in the ad group is not relevant to the keyword, this again might benefit from being moved to different ad group with a better targeted ad.
You cannot really move the keyword, but you will have to copy and paste the keyword to the new adgroup. Then you can either delete or pause the original keyword in the previous ad group. The user interface does not provide a move operation (I think), most probably just to avoid the confusion about the associated statistics.
When you edit, move or do pretty much any operation on a keyword, the associated statistics is reset. This means that when you copy (ie, move) the keyword to another ad group, none of the previous stats are carried over.