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How Time-Based Graphs Can Deceive You, And Why You Should Run Real Experiments

Tagged As: Optimization

Occasionally I see how well-intentioned people try to improve their online business by using graphs in a misleading way. With access to powerful analytical tools (ex: Google Analytics), designers, business owners and marketers hunger to increase the metrics displayed on their beautiful graphs – a good thing. Unfortunately many are lured into a sort of time-based analysis for their decision making. The common and deceiving practice involves making a site-wide change followed by a comparison of some conversion rate before and after the day of the change. Such an approach although intuitively correct may be highly deceiving as it lacks sensitivity. In reality there are just too many outside forces acting on your graphs that will pollute your analysis. It is therefore superior to setup a properly controlled experiment instead of doing time-based comparisons.

Time-Based Comparison: In A Perfect World

Intuitively it makes sense to roll out a change on your site and then hope to see its effect in a graph, a day, a week or a month later. Given that most people have access to GA for their site, this may be the simplest, cheapest and fastest way to validate an improvement (or so some might believe). Unfortunately such a comparison only works in graphs which have rather stable data, such as the one below:

Best Tests
The perfect world scenario: daily conversion rates visibly affected by a change

Time-Based Comparison: In The Real World

In the real world data isn’t flat or stable. Real data has variations, irregularities, spikes, and fluctuations and this is where our time-based comparisons break down. Take a look at a real data sample snippet below which contains 2 months of sales. In the first month the conversion rate was 9.16% and in the second month the conversion rate moved up to 9.84%. Please keep in mind that no known changes were implemented during these two time frames and yet there was a relative increase of +7.42% from pure uncontrolled fluctuation.

Best Tests
The real world scenario: daily conversion rates (sales) for an unchanged control containing fluctuations

Deceiving Winners & Losers

Now imagine if we were to introduce a change somewhere in the middle between the end of the 1st month and the start of the 2nd month. Let’s say that it’s a negative change with a -5% decrease to sales. We would first have to make an assumption that without any changes the conversion rate shouldn’t change between one month and another (and remember that it did). Continuing with our faulty assumption we might then compare the first time frame (before the change) to the second time frame (after our change). Seeing any increase in the graph we might keep the change, or roll back to an older version of our site if we observe a decrease.

In this scenario our hurtful change of -5% to sales would actually be covered up by the positive +7.42% fluctuation in the second month. In other words, we would be deceived by an observed +2.42% impact on sales (containing our -5% change) and would decide to keep it.

How Common Are Fluctuations Between Time Frames?

To answer how common such conversion rate differences or fluctuations might be, I’ve selected 5 data samples from 5 different sites. All shown examples below have relative fluctuations ranging between 5% and 26% suggesting that sable graphs are hard to find.

Metric 1st Conversion Rate 2nd Conversion Rate Uncontrolled Difference
Software Sales 9.16% (1 month) 9.84% (2nd month) +7.42%
Subscription Sales 24.1% (1 month) 20.6% (2nd month) -16.7%
Insurance Quotes 37.1% (1 week) 41.3% (2nd week) +10.8%
Online Course Leads 7.78%% (1 month) 7.35% (2nd month) -5.46%
Ebook Sales 2.12% (1 month) 2.68% (2nd month) +26.39%

A Real Controlled Experiment

To compensate for all variables beyond our control twisting our data over time, a simple controlled experiment is needed (ex: A/B test). In a real controlled experiment both the A and the B are shown to random visitors for the same time frame. Hence they both ride out any fluctuations driven by external factors. So if we have a spike sale, our controlled experiment is immune. If Bob breaks the site, our experiment is immune. If Jenny makes some additional changes to the site (for good or bad), our experiment is immune. Here is the same real graph with both the A (control) and the B (variation) charted out – notice how both A and B follow a similar trend (from things which we might not even be aware of).

Best Tests
A real controlled experiment: both A and B have the same time frame

Time-based graph analysis lacks sensitivity. Such an analysis would only work if our introduced changes are larger than the fluctuation between the different times being compared. Controlled experiments on the other hand have more sensitivity to detect any effect size, given enough time (and statistical power).

A/B Testing Tools

If you need a powerful a/b testing tool which is cost-effective and reliable I would recommend Visual Website Optimizer as a start. If on the other hand you have deep pockets, a really polished tool is Optimizely. We’ve also been recently seeing experiments setup using Convert and liking it as well.


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Posted by Jakub Linowski on Dec 31, 2017

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Jakub LinowskiDmitriy Melihov Recent comment authors
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Dmitriy Melihov

Thank you for the hint. How about trends over time? If we start to make updates on some day and see stable increasing graphs over 1-2-3-4-5-6 months? I’m worrying because there are a lot of websites with low traffic where we can’t make meaningful A/B comparison