From my agency days springs the expression “Garbage in, garbage out” used in video and audio production meaning essentially, the best (audio / video product) quality comes from the best quality source material.
Sure, you could hide the occasional aircraft noise in a perfect 20 minute interview, but you couldn’t get pristine out of an interview at Grand Central Station with system announcements every 3 minutes.
Problems in post production were almost always solvable by exact and precise preparation before a shoot or audio session.
Same goes for web analytics.
Using Google Analytics as an example, having distinct goals and pathways defined to measure success will always give better and more actionable results than poorly planned goal implementation (or not doing it at all!)
Clients without transactional (i.e. purchase) goals on their sites often omit goal setting as they only believe the relevance is to dollars and cents results. Goals can be as simple as ensuring certain content is reviewed, or comment is left, or a click through is obtained (it could be between pages or sites or specific exit points.)
Data is the most valuable aspect of web analytics, setting real and relevant goals is a necessity in any search engine optimization campaign.
Dirty data in = dirty data out = waste (garbage or rubbish) of time.
Take out the garbage and benefit from the clean, fresh (and valuable) data!