Standard Metrics Revisited: #6: Daily, Weekly, Monthly Unique Visitors.

By Avinash Kaushik

bright purple Do you have a sneaking, yet unshakable, suspicion that your Web Analtyics Vendor is sometimes just trying to mess with you?

Guess what?

It's true!

All web analytics tools have a smattering of metrics and key performance indicators that were created just because someone decided it would be cute to add / subtract / multiply / divide some numbers.

Many of these don't pass the first sniff test and when if they do you are still left wondering: "What in God's name and all that is holy in this world am I supposed to action based on this metric?"

The answer?


With that gloriously upbeat set up let me tell you what we are going to cover today: Three metrics that are available in pretty much all "adult" web analytics tools. Daily, Weekly, Monthly Unique Visitors.

daily weekly monthly unique visitors They are so common yet most people don't understand them well enough and fewer still realize how harmful these can be to your health even in day to day use.

So in this post we try to understand the most basic of the web analtyics basics, the Unique Visitor computation.

What's a Unique Visitor?

It is simple really. . . .

Technical Definition: Count of all the Unique cookie_id’s during a given time period.

English Definition: The first time someone visits your site a first party persistent cookie is set in their browser. This cookie lasts any where from several months to several years. Each time that person visits your site that cookie identifies them as the same browser.

unique visitor really!Notice I said browser, not person. It is likely, but not always true, that each a unique visitor is a unique person.

You can learn a lot more about Visits and Unique Visitors in this post: Standard Metrics Revisited: #1: Visitors.

Very predictably every 18 months or so the blogosphere goes wild with how accurate, or not, the Unique Visitor metric is. Much mud is thrown around. Indignations are foisted on the world. Name calling ensues.

Regardless of that Unique Visitors remains a valuable metric that used correctly, in place of Visits, measures success of your online marketing efforts.

Oh and your best weapon against ignorance? Education. See above post on Visitors. And this one: A Primer On Web Analytics Visitor Tracking Cookies. It covers cookies and deletion rates and other such yummy stuff. Read that and you have my word you'll be the smartest cookie in the room.

See what I did there? :)

Daily, Weekly, Monthly Unique Visitors:

In many web analytics tools (say Yahoo! Web Analytics, Omniture, WebTrends etc, but not in Google Analytics ) you'll also see Daily Unique Visitors, Weekly Unique Visitors, Monthly Unique Visitors and, sometimes, Absolute Unique Visitors.

monthly trend of daily unique visitors

Each is trying to tell you something about Unique Visitors, yet if you pause and think about it, I mean really pause and think about it, you'll realize two of these are really bad for your health, and the third should be used with caution.

The core reason is that what looks attractive initially becomes progressively worse as you extend the time period. The Daily metric, so to speak, does not even last in value beyond two days!

So let's spend a second understanding this slightly yucky phenomenon.

Here's the data, from, where WebTrends is used for tracking Visitors. . .

visits by unique visitors

Now let's go measure the complex set of metrics that'll stare at you, let's say when you crack open Omniture or WebTrends (or pretty much any other competitive web analtyics tool).

The Web Analytics Unique Visitors Story:

Before that realize that what you see will depend on the time period you are looking at. [Arrrh!]

And before I really really jump in… you'll see a metric called Absolute Unique Visitor. I am going to use that as a proxy for how unique visitors should be computed correctly, regardless of what time period you are computing it for. Keep an eye on that number.

Looking at Month 1 and Week 1 at the end of Day One:

daily unique visitors

If you ran your reports at the end of day one here is what your analytics tool will report to you, with some delight and joy I might add. . .

Daily Unique Visitors: 3 Weekly Unique Visitors: 3

Monthly Unique Visitors: 3

Makes sense right? Do a happy dance, high five someone next to you, heck give them a hug and a kiss.

Now let's make this more "complicated".

Looking at Month 1 and Week 1 at the end of Day Two:

unique visitors for two days of a week

If you ran your reports at the end of day two here is what you'll see. . .

Daily Unique Visitors: 5 Weekly Unique Visitors: 3 Monthly Unique Visitors: 3

Absolute Unique Visitors: 3

Slow down the happy dance a bit.

Note the silly effect on Daily Unique Visitors, even though it was the exact same folks, Dennis and Matt, from the earlier day who visited on day two. They get counted twice.

Life lesson: Daily Unique Visitors is a useless number if you are looking at a time period of more than one day!

Let's keep going.

Looking at Month 1 at the end of Week One:

unique visitors at the end of week one

Crack open your analytics tool, it has been a long week, look at the metrics, here's what you'll see. . .

Daily Unique Visitors: 6 (!) Weekly Unique Visitors: 3 Monthly Unique Visitors: 3

Absolute Unique Visitors: 3

Note the continuing uselessness of the Daily Unique Visitor number (and even if you trend it over time, as in the blue graph above, analyze what it is actually showing you? what's the insight?).

In your Web Analytics Tool you might see a report that looks like this:

summing daily unique visitors-no!

By know you know why there is a sad frowny face in that last Total row. Right?

Repeat: Life lesson: Daily Unique Visitors is a useless number if you are looking at a time period of more than one day!

Looking at Month 1 at the end of Week Two:

weekly unique visitors

Gather everyone in your close proximity in the office, form a circle, hold hands, close your eyes, say a quite prayer, now open your analytics tool. . .

Daily Unique Visitors: 10 (!!) Weekly Unique Visitors: 6 (!) Monthly Unique Visitors: 5

Absolute Unique Visitors: 5

The Weekly number is wrong because it counts: Avinash, Dennis, Matt, Matt again, Ian and Jim. It counts Matt again because he visited during both weekly time periods.

Life lesson: Weekly Unique Visitors metric is useless if you are looking across multiple weeks. We've covered above why Daily Unique Visitors is, to put it mildly, sub optimal.

Ok only two more scenarios left, hang in there, it gets better.

Looking at the end of Month 1, for the whole month:

monthly unique visitors

By now I am sure you are 100% up to speed on what you are going to see. . .

Daily Unique Visitors: 13 (!!!) Weekly Unique Visitors: 9 (!) Monthly Unique Visitors: 6

Absolute Unique Visitors: 6

There is now triple or double counting happening in both the Daily Unique Visitors and Weekly Unique Visitors numbers.

Life lesson: Both Daily Unique Visitors and Weekly Unique Visitors numbers are useless when you look at a time period of a month.

One last scenario, not to make your brain hurt but rather to ensure you reach the state of maximum Analysis Ninja enlightenment!

Looking at the end of Month 2, for the two months:

visits by unique visitors

Tingling with excitement. . . here's what you'll see. . .

Daily Unique Visitors: 19 (kill me now!) Weekly Unique Visitors: 15 (can't breathe!) Monthly Unique Visitors: 12 (!)

Absolute Unique Visitors: 9

There is now triple or double counting happening everywhere, the Daily Unique Visitors, Weekly Unique Visitors and Monthly Unique Visitors numbers.

The correct measure of unique is the Absolute Unique Visitors metric because it de-dupes the unique visitors across the entire time period you are reporting on.

Life lesson: Both Daily Unique Visitors and Weekly Unique Visitors numbers are totally really useless when you look across months. Use Monthly Unique Visitors with caution, knowing it is only de-duping for each month and then summing the number for each month.

absolute unique visitors

If your tool provides Absolute Unique Visitors you are in luck because then you are getting true unique visitors across whatever arbitrary time period you choose.

Google Analytics provides you with the Absolute Unique Visitors metric.

[Update: In the new version of Google Analytics this metric is called Unique Visitors. Everything about it described is the same in terms of it providing a unique de-duped count, it's just called Unique Visitors. You'll find it in standard reports, and you can easily add it to any custom report. Try it!]

google analtyics true unique visitors across time periods

It will do that across set time periods, like the month of March (or any number of months). . .

march unique visitors

or across arbitrary time periods, as Monday March 9th through Thursday March 19th. . .

random date range unique visitors

It will dedupe the numbers when it reports to you, rather than adding the totals of each day, week or month.

Complex but bonus for Ninjas: Depending on which graph you look at, daily, weekly or monthly, it will intelligently compute the number for each time period and also show you the aggregate deduped number for that time period.

Fly in the otherwise rather healing ointment?

Google Analtyics does not compute Absolute Unique Visitors when you segment the data, when you use the Advanced Segmentation feature. Those of you who read the blog know my utter infatuation with segmentation, so you can easily understand how sad this makes me.

You can get Absolute Unique Visitors for segments by using the "create a filtered profile that just data for the segment" method and that works if you have forethought. But it is sub optimal, just like some "enterprise" web analytics vendors telling you that you can only segment if you tell them before the fact what you might want to segment later.

Why do Web Analytics Vendors torture you with Daily, Weekly, Monthly Unique Visitors?

why so painfulI knew you were asking yourself this question!

Good on you Mate.

If these metrics are that sub optimal, why do web analytics vendors put us through this torture?

Simple: Compute power (translation: cost, for them).

It is very computationally intensive to calculate for you the true real (Absolute) Unique Visitor number across any arbitrary time period or across multiple weeks or months.

Increased computational intensity for the vendor means more processing time and higher costs.

So doing Daily, Weekly and Monthly counts (and then summing them up) is cheaper for them.

After the first vendor decided to do this, and there were no major outcry from Web Analytics Users (or even Ninjas!), others quickly followed.

For the more prevalent vendors in the space Google Analytics is one the rarest that provides the truly de-duped Absolute Unique Visitor metric (in aggregate, not segmented, boo!). Only time will tell when Google will buckle under the computation/cost weight and stop providing it true Absolute Unique Visitors.

[Update: Both NedStat and Xiti, two wonderful European companies do allow for computation of Absolute Unique Visitors out of their standard packages, no additional payment or gyrations required. Add Unica's NetInsight to that list as well! Hurray!!]

There are some vendors that will tell you that you can buy their more expensive data warehouse solutions (at an additional cost on top of what you pay today) and then compute Absolute Unique Visitors yourself. True. Ask for the cost. Ask if its really Absolute. If prudent, pay more. Regardless, be informed.

Long lesson.

But now you are truly at a Analysis Ninja black belt level of proficiency!

Now your turn.

Please share your comments / feedback / critique / hugs / non-hugs about this post. What does your tool do? How do you think we should improve things? What would you eliminate? What would you add? What did I miss?

Couple other related posts you might find interesting: