In what follows, we list four different mistakes marketing managers are prone to make when assessing the return on their advertising investments (ROAS), and the only ways to avoid them.

Google, Facebook and the other platforms are all too happy to claim success for a conversion. In many cases, adding up the data from the different platforms will show up as multiple purchases while in reality there was only one.

It’s only through the use of their CMS systems, or possibly even just their Google Analytics last touch attributions, that businesses will avoid over-counting purchases.

The most mundane reasons can cause spectacular changes in patterns. Take the weather, when a period of cold and rain followed by agreeable weather will have people run to be outside and forget about online purchases for a few days. Does such a scenario indicate a failure of the advertising campaign? No, of course not.

Businesses will do well to focus on year-over-year measurement, comparing one January to the next, if they want to measure the success of their ecommerce marketing successfully.

Google Ads cookies used to expire in 30 days. Most ecommerce marketers got into trouble when Google’s data was compared with the data from other channels. It is important to be able to compare apples to apples—or, when you must compare oranges to apples, to at least know how many apples an orange is worth.

Another mistake pertains to plain errors in setting up the data sets. Google Analytics is very useful for measuring ROAS if, and only if, the team actually knows how to use it.