Until about five years ago, most users accessed the web via a single device, so all you needed to do to measure conversion rate was divide conversions by visits. With tools such as Google Analytics providing a simple method to calculate this important metric, marketers were able to move away from engagement-based metrics such as page views and time-on-site, and bring an unprecedented degree of rigor to how they measured site performance and marketing efficiency. 

Today the scenario is more complicated. The rise of tablets and smartphones means that consumers are now able to interact with sites through a multitude of devices, and conversions can occur on any of those devices. In this multi-device world, the way we calculate conversion rate needs to be altered because as things stand it is not possible to evaluate conversion rates across devices when comparing performance. A new approach is required to provide a true picture of the contribution made by each device type to overall performance.

Mobile and the rise of multi-device users

We first became aware of this insight when a few of the businesses we work with approached us to help with understanding their conversion data. Over the past ten years, tools like Google Analytics have kept pace with the emergence of new technologies, so we could easily break out conversions by device. What we noticed was that while the total share of conversions for desktop devices was shrinking, the conversion rate for desktop devices continued to increase. This felt counter-intuitive: with mobile and tablets gaining ground year by year on desktop in total visits and conversions, why would the rate at which desktop visitors convert continue to rise while the rate for mobiles remained relatively static?

The answer to this question lay in the differing behaviour of users on each device. Mobile devices are a big growth engine for new visitors, and some of those visitors are converting on mobile, but there is a significant proportion of users for whom mobiles and tablets are primarily a research and discovery tool. If you look at your analytics data and notice a large number of new desktop users who convert quickly, with below-average session pageviews, there’s a good chance that what you’re actually seeing are users who have thoroughly researched your products or services on another device, and then switched to their desktop to convert, preferring the ease of use of the desktop experience. But desktop conversion rate is also growing because traffic is a factor in conversion rates. If your traffic is dropping faster than your conversions, your conversion rate will go up.

Introducing ‘Multi-Device Conversion Rate’

Comparing in-device conversion rates in a multi-device world can be misleading. Mobile devices that have done the hard work of reaching and informing potential customers are not given credit for their contribution to conversion, while desktop devices see their conversion rates rise by virtue of their better user experience coupled with the impact of being used less as a research device. Mobile looks like it is under-performing while desktop is soaring, concealing the essential contribution of mobile to growth in both visitors and customers. If you’re relying on in-device conversion rates, you could underestimate the growing importance of mobile, ignoring its vital role as your modern-day digital store-front and starving it of investment with damaging consequences for your business strategy.

So, how do you adjust to this new reality and reveal the conversion contribution of each device type? Recently, we’ve started using a new metric to try and capture the true conversion growth of all devices: Multi-Device Conversion Rate.

This new metric enables measurement of device conversion side-by-side and on an even playing field, by calculating individual device conversions as a percentage of visits across all devices. Mathematically, this makes it possible to compare conversion rates between devices as they are all calculated off the same base. It also means that it is possible to see how each device contributes to the overall conversion rate across all devices as it is now possible to add all the conversion rates together. By looking at these percentages over time, i.e. from one year to the next, it is possible to see the true device contribution to multi-device conversion rate growth or decline. The focus should always be on growing the conversion rate across all devices as this is the metric most similar to the conversion rate of old when all traffic and conversions happened on the same device and it best compensates for user movement between the devices.

Looking at conversion rate across all devices goes hand-in-hand with looking at cost per acquisition (CPA) across all devices. Many marketers have individual CPAs for each device but this is a flawed strategy as users acquired on one device might convert on another. In essence, a device based CPA states “we care which device users convert on.” By having a cross-device CPA that statement alters to “we care that users convert.” This second statement often leads to greater conversions and ROI as it allows a higher CPA on one device to be compensated for by a lower CPA on another device. Because desktop continues to represent the lion’s share of transactions, a big percentage increase in mobile marketing cost can be offset by a smaller percentage increase in desktop revenue. And, given the predominantly assistive nature of mobile, allowing more traffic through higher CPAs will likely lead to more transactions on desktop.

Question your questions

Easy access to calculated metrics has transformed how marketers optimise performance, but it’s important to remember that as technologies evolve, so must the means we use to measure them. 

Multi-device conversion rates are a device-level calculation to help you better understand the contribution of each device toward your true conversion rate across all devices. For Google AdWords users who would like more granular cross-device data we recommend using the cross-device conversion column in their AdWords account, available at the campaign and ad group level. To learn more about how to access and use this data, check out the Google AdWords help centre.