I did a little experiment to see if people on Twitter could spot the conjunction fallacy in a specific statement.

I set the scene by describing the common traits of a fictional user; a React.js developer who uses Apple products. I then ask a simple question, Which is more probable? Is it that they use a laptop, or they use a MacBook Pro? The results were favourably towards MacBook Pro ... by a lot.

That is the incorrect answer.

Another way of writing this would be:

  1. Sam uses a laptop
  2. Sam uses a laptop which is a MacBook Pro

Here, we've introduced a conjunction - it's a laptop and a MacBook Pro, compared to it being just a laptop.

Since the probability of it being both a laptop and a MacBook Pro must be equal or less than it just being a laptop, the correct answer is that Sam uses a laptop. There is no scenario where Sam uses a MacBook Pro which is not a laptop.

This is a well known puzzle called the Linda Problem[1] which is summarised as follows:

The probability of two events occurring together (in "conjunction") is always less than or equal to the probability of either one occurring alone

Conjunction fallacy in your personas

When identifying personas it's easy to make this assumption about your users and be too specific. Take this example for an online store:

Sam loves shopping online and is always on the hunt for the best bargain. Often checking our voucher sites and Groupon, they love to hunt for getting the best deal and will often spend more if there's money off.

Now you're tasked with designing the checkout process for this online store. Reading that persona you come to the conclusion that there needs to be a place for a voucher code to be entered during the checkout. You design a nice voucher call-to-action and Sam is super happy.

Here is the conjunction fallacy, so let's ask, which is more probable?

  • Sam will purchase on your online store
  • Sam will purchase on your online store and has a voucher code

The persona suggests the latter, but what you've successfully done is created a complication for the set of users who don't have a voucher code. Studies have shown that up 27% of people would exit your site and go searching for a voucher code[2].

Be specific, but not at the detriment of others

You want to be aware of how true and probable your personas are. It's okay to improve the experience for a subset of users (those who have voucher codes), so long as it isn't to the detriment of the superset (anyone buying on your site).

Timing is everything

I discovered the Linda Problem reading When: The Scientific Secrets of Perfect Timing by Dan Pink. It turns out that people are most vulnerable to the conjunction fallacy in the afternoon than in the morning.

My advice, design your personas in the morning.

References

  1. Conjunction fallacy and the Linda Problem - Wikipedia
  2. The Sad Tale of Abandoned Shopping Carts - eMarketer