As we saw in Chapter 9 - Recruiting Participants, you may have separate tests set up for separate user groups. You can then use several methods (separate email lists, descriptive links, screening tools) to direct each participant to the right test.

But what if you have several tests that all target the same users? This is actually quite common, because we often want to test 2 or 3 alternative trees against each other to see which ideas work best.

There are two basic ways of splitting your participants randomly among tests – using mutually exclusive links and using code.

The simplest way to split participants randomly is to set up your tests as a list of links, then construct some arbitrary way of getting participant to pick different links.

We usually do this by asking participants to choose the link that corresponds with the first letter of their first name. If we are running 3 tests, for example, the first link is A – I, the second is J – P, and the third is R – Z:

• ss from MoE, etc.

If we want a weighted split (e.g. 70/30) for some reason, we can adjust the letter ranges accordingly. Midway through the test run, if we notice that we’re not getting enough people on test 3 (for example), we can expand its letter range to help it catch up.

Beyond being easy to set up, this method is also guaranteed to work in all browsers (for web-ad explanation pages) and all email clients (for email invitations), because it does not rely on JavaScript (see below).

A more elegant way to split participants randomly is to use a bit of JavaScript code. You present a single link to all participants, but when the link is clicked, it triggers some JavaScript that picks a random number (e.g. 1, 2, or 3 if you’re running 3 tests), then directs the user to the corresponding test address automatically.

Here’s how it looks in a web ad’s explanation page:

• ss
  

…and here’s the JavaScript code for the link:

~code

This works well in an explanation page on your website (that your web ad would direct to), but it may not work reliably in email invitations because some email clients might disallow JavaScript in incoming messages. ~check this

As a workaround, you could make your email invitation go to a web page on your site that shows a “One moment please…” message, does the random-test-by-code selection, then automatically redirects to the corresponding test address.

Next: Launching the test(s)