“Test one variable at a time” is a lie

It’s not an intentional lie and its heart is in the right place, but it’s wrong nonetheless.

The reason people will tell you to test only one variable at a time is that you want to be able to isolate why what happened happened.  So, for example, if you changed the teaser on an envelope and sent it to an equivalent audience at the same time with the same contents in the envelope, if there was an increased response rate, that is a winning test because of the envelope.

This is a fine way to test if there’s only one thing you want to learn at a time.  You can refine your program this way, getting better and better.  This is the direct marketing equivalent of kaizen – the practice of continual improvement popularized in manufacturing, but now applies to much strategic thinking.

But there are some significant problems with this:

  • You can’t test synergy between variables. Let’s say you have a subject line you’d like to test.  However, it may work better with a different version of your email; after all, you wrote the original subject line for this email – the new one may not fit as well.  Testing one thing at a time may not allow us to test the most coherent versions of each of your offers.
  • It can lead to small ball, where you only test things at the most granular level. In his book Fundraising When Money is Tight, Mal Warwick talks about testing teaser copy 25 different times with almost as many clients.  Of the tests, 21 – 84% — showed no difference (and these were at quantities that would have shown a difference had there been one).  This is an OK learning if you can learn other things from the package as well, but if that’s all you learn, you’ve investing in testing without any return more than four out of five times.
  • It can’t make significant leaps forward. Let’s say you have a control piece in decline.  You know it needs to be replaced because of its response rate.  Or maybe, in a more positive outlook, you’ve accomplished the goal you were striving toward.  Either way, the way to get rid of this piece isn’t to test the envelope one year and the response device the next year – you have to test more than one variable at once.

All in all, this violates a rule you should have for yourself – to learn as much as possible whenever possible.  Think of it as if you were trying to reach the highest elevation on Earth.  If you had the rule of “go up from where you until you can’t go up any more,” you will reach a peak higher than you are currently, but by no means the highest point possible.  Similarly, if you had the rule “climb to the highest point you can see, even if it means going down a bit,” you will be doing better and getting higher than you were, but this iterative process will not lead to you having to don an oxygen tank anytime soon.

So it is with testing.  Testing one variable at a time will get you closer and closer to your local maximum, but not the global maximum.

But the basis of the argument for variable isolation is not untrue.  You still need to be able to figure out what works and what doesn’t.  The trick is sussing out what did what in your test.  That’s what we’ll cover tomorrow: how to layer multiple single tests to get results you can act on.

“Test one variable at a time” is a lie

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