Since my last post­ing about unique vis­i­tors and vis­its, I’ve received a great deal of feed­back. Much of this feed­back praised my blog, with one reader say­ing it was “a voice of rea­son” in an oth­er­wise “ill informed web ana­lyt­ics indus­try”. That may be a bit strong, but I love it nonetheless!

As might be expected, my post was also met with some push­back — which is also great. I’d hate to post some­thing every­one agreed with…what’s the point of that? Now these folks had some ques­tions and indeed some mis­un­der­stand­ings that I wanted to clear up.

So the theme of this post­ing is sim­ply that all unique vis­i­tors are not cre­ated equal.

To some of you, this may be com­mon sense. To oth­ers, this is new infor­ma­tion. And to oth­ers, this is purely hereti­cal. “How dare you ques­tion my unique vis­i­tors?! Such blasphemy!”

I know, I know…I’m always rock­ing the boat. But the web ana­lyt­ics indus­try is so young and clouded by mis­per­cep­tions that some­one needs to step in and put a stake in the ground. And why not me? So here it goes…

Rather than hav­ing an emo­tional debate about the accu­racy of unique vis­i­tors and whether you should or shouldn’t use them, let’s start with some basic observations:

  • Fact #1: Most web ana­lyt­ics sys­tems rely upon cook­ies to iden­tify unique visitors.
  • Fact #2: Cook­ies are set at the browser-level by domains like omni​ture​.com.
  • Fact #3: If cook­ies are not accepted or used by a domain, web ana­lyt­ics sys­tems mostly rely on IP+user agent com­bi­na­tions and/or uni­ver­sal vis­i­tor iden­ti­fiers (i.e. a reg­is­tered user ID)

Now assum­ing you agree with those obser­va­tions, let’s look at 15 sce­nar­ios that impact the accu­racy of your unique vis­i­tors counts:

  • Sce­nario 1: One per­son uses mul­ti­ple browsers from one com­puter from one location
  • Sce­nario 2: One per­son uses mul­ti­ple browsers from mul­ti­ple com­put­ers at mul­ti­ple locations
  • Sce­nario 3: Sev­eral peo­ple use one com­puter with one browser from one loca­tion (i.e. a typ­i­cal 4 per­son household)
  • Sce­nario 4: Sev­eral peo­ple use one com­puter with sev­eral browsers from one loca­tion (i.e. a typ­i­cal 4 per­son household)
  • Sce­nario 5: Sev­eral peo­ple use sev­eral com­put­ers with sev­eral browsers from one loca­tion (i.e. an increas­ing trend among households)
  • Sce­nario 6: One per­son deletes cook­ies for one browser
  • Sce­nario 7: One per­son deletes cook­ies for mul­ti­ple browsers
  • Sce­nario 8: Your web ana­lyt­ics pack­age reports both cook­ied and non-cookied users in the unique vis­i­tor counts
  • Sce­nario 9: Your web ana­lyt­ics pack­age doesn’t use cook­ies or reg­is­tered IDs to iden­tify unique visitors
  • Sce­nario 10: One per­son rejects cook­ies on one browser
  • Sce­nario 11: One per­son rejects cook­ies on one browser, but accept cook­ies on another (for exam­ple, Inter­net Explorer vs. Firefox)
  • Sce­nario 12: Many peo­ple from one com­puter reject cookies
  • Sce­nario 13: One per­son rejects cook­ies from one com­puter but accepts cook­ies on another computer
  • Sce­nario 14: IP pool­ing by major ISPs like AOL means many IP addresses for one vis­i­tor as they move from page to page on your website
  • Sce­nario 15: Ded­i­cated Cor­po­rate IP addresses and stan­dard­ized browser con­fig­u­ra­tions mean one IP address for many people

At any given point in time, all of these sce­nar­ios are play­ing out on your web­site and are inher­ent in your unique vis­i­tor counts. Whether you like it or not. And just for the record — these are not ven­dor spe­cific issues as one of my read­ers sug­gested. Hope­fully you can see that every sce­nario I’ve listed above has lit­tle to do with your web ana­lyt­ics plat­form. Rather, they are byprod­ucts of web mea­sure­ment in general.

So what is your def­i­n­i­tion of a unique vis­i­tor? How many “uniques” do you really have? More impor­tantly, how many “peo­ple” or prospec­tive cus­tomers are you really reaching?

If you get 1 mil­lion unique vis­i­tors in a given month, is that 1 mil­lion oppor­tu­ni­ties to sell? Or is it just 500,000? Maybe Sce­nario 3–5 are really heavy in your site traf­fic mix, and you have 1.5 mil­lion unique “prospects” that you could convert.

No mat­ter what the case, given all 10 sce­nar­ios above, what num­ber would you feel con­fi­dent report­ing back to your CEO? Can you have con­fi­dence in a unique visitor-based con­ver­sion rate? Or even a unique vis­i­tor count? Some of the sce­nar­ios I’ve listed above will inflate your unique vis­i­tor counts and some of the sce­nar­ios will decrease it. Some will actu­ally do both! And it’s nearly impos­si­ble to mea­sure the net impact of all these factors.

For these rea­sons, as I sug­gested in my first post on Vis­its and Unique Vis­i­tors, I almost always use Vis­its as a strate­gic mea­sure of how well my sites are con­vert­ing prospects. If noth­ing else, each Visit rep­re­sents an oppor­tu­nity to con­vert a prospec­tive cus­tomer. It’s no more com­pli­cated than that. And because vis­its (or ses­sions) are gen­er­ally mea­sured based on cookied-visitors only, they are much more accu­rate than unique vis­i­tors (vis­its are in effect a sub­set of unique visitors).

Vis­its, by def­i­n­i­tion, also do not require you to deter­mine “unique­ness”. In other words, all Vis­its are cre­ated equal.

Of course, rely­ing on vis­its has draw­backs in mar­ket­ing analy­sis, seg­men­ta­tion, latent con­ver­sion, life­time value, etc, etc. But that’s OK because by the time you get to these draw­backs, you shouldn’t be focus­ing on unique vis­i­tors any­way — you should be focus­ing on unique cus­tomers.

So what is a unique cus­tomer? Why is it bet­ter than unique vis­i­tors? And why won’t it suf­fer from the same inac­cu­racy issues? As much as I’d love to dive into that now, you’ll have to stay tuned until my next post — I’m all out of time today. In the mean­time I wel­come your feed­back and as always, if you’d like assis­tance under­stand­ing how to lever­age web ana­lyt­ics to max­i­mize ROI, please do not hes­i­tate to con­tact me and the Omni­ture Best Prac­tices Group.


I do actually, although as you might imagine I can’t mention their names. I also did a similar analysis myself when I was a customer of analytics many years ago. This was well before the unique visitor/cookie debate, I was actually more interested in understanding multi-channel behavior. In either case, my exposure to these initiatives has confirmed one thing – variances in the accuracy of unique will differ by website. There really isn’t a hard and fast number you can use to estimate it. Quite frankly I’ve seen numbers in the low single digits, all the way up to high double-digits. All of these are based on a registration-based analysis of uniques. If you don’t require registration on your site, look to some of your peers in Media or eCommerce, as registration is quite pervasive there.


Hi Matt. Do you know of any companies who require registration that have compared their Omniture uniques against their more accurate registration logs to measure how off the former numbers might be? Thanks