Identifying the 20%
Export a campaign report and then sort high to low by conversions. Do a little division in the cells next to clicks, impressions, cost and conversions to see the percent of the total each campaign contributes.
In my example you can see that Campaign 1 contributes 33.8 % of total conversions for the account yet it has the lowest click through rate and it’s low impression share is almost all due to rank. There is some low hanging fruit to be had with this campaign by adding negative keywords, improving adtext and adjusting bids.
If you didn’t think to start working on the 20%, or most important first, you might be tempted to start working on Campaign 14 which has the highest cost/conversion. You may not realize that even though it is performing badly, it’s only contributing .4% of total spend. All the effort on that campaign would yield little help to the account as a whole.
The moral is work smarter not harder.