I would have thought that a lot of this stuff was obvious, but, judging from my experience working with many different companies, it’s not. All of the examples here are real mistakes I’ve seen made by smart, reasonable, employed people. A few identifying characteristics have been changed to protect the innocent, but in general they were product owners, managers, or director level folks.
This post only covers mistakes made in analyzing quantitative data. At some point in the future, I’ll put together a similar list of mistakes people make when analyzing their qualitative data.
For the purposes of this post, the quantitative data to which I’m referring is typically generated by the following types of activities:
- Multivariate or A/B testing
- Site analytics
- Business metrics reports (sales, revenue, registration, etc.)
- Large scale surveys
Statistical SignificanceI see this one all the time. It generally involves somebody saying something like, “We tested two different landing pages against each other. Out of six hundred views, one of them had three conversions and one had six. That means the second one is TWICE AS GOOD! We should switch to it immediately!”
Ok, I may be exaggerating a bit on the actual numbers, but too many people I’ve worked with just ignored the statistical significance of their data. They didn’t realize that even very large numbers can be statistically insignificant, depending on the sample size.
The problem here is that statistically insignificant metrics can completely reverse themselves, so it’s important not to make changes based on results until you are reasonably certain that those results are predictable and repeatable.
The Fix: I was going to go into a long description of statistical significance and how to calculate it, but then I realized that, if you don’t know what it is, you shouldn’t be trying to make decisions based on quantitative data. There are online calculators that will help you figure out if any particular test result is statistically significant, but make sure that whoever is looking at your data understands basic statistical concepts before accepting their interpretation of data.
Also, a word of warning: testing several branches of changes can take a LOT larger sample size than a simple A/B test. If you're running an A/B/C/D/E test, make sure you understand the mathematical implications.
Short Term vs. Long Term EffectsAgain, this seems so obvious that I feel weird stating it, but I’ve seen people get so excited over short term changes that they totally ignore the effects of their changes in a week or a month or a year. The best, but not only, example of this is when people try to judge the effect of certain types of sales promotions on revenue.
For example, I've often heard something along these lines, “When we ran the 50% off sale, our revenue SKYROCKETED!” Sure it did. What happened to your revenue after the sale ended? My guess is that it plummeted, since people had already stocked up on your product at 50% off.
The Fix: Does this mean you should never run a short term promotion of any sort? Of course not. What it does mean is that, when you are looking at the results of any sort of experiment or change, you should look at how it affects your metrics over time.