Core Web Vitals: Field Data vs Lab Data

It’s June 2021 and Google has finally begun rolling out the long awaited Page Experience update. If you’re wondering if Page Experience update maters, we got you covered. But if you’re struggling to understand what Field Data and Lab Data are and why they’re different, this is the article for you.

Ever done a PageSpeed Insights test and wondered why your mobile scores are much lower than your desktop scores? After all, aren’t both mobile network speed and device processing power supposed to be catching up with desktop? Read on to see what causes your poor mobile Core Web Vitals scores, why it may not matter as much as you think and why you should fix it regardless.

To understand why your mobile Core Web Vitals scores are lower, you need to understand the difference between lab data and field data. Let’s use Amazon homepage PageSpeed Insights assessment to walk through the differences between the two.

Core Web Vitals: Field Data vs Lab Data
Core Web Vitals: Field Data vs Lab Data

You can also look at the results here.

Field Data

The blue rectangle is where the Field Data is, the green one has the Lab Data. As you can see from the screenshot, Field Data looks great, Lab Data has some issues with Largest Contentful Paint and Cumulative Layout Shift. Does this mean the page is failing? It depends on who you’re asking, but if it’s Google and Page Experience you’re worried about, with a page like this you’re doing great.

Looking at LCP, you can see that it’s 87% “good”, 8% “needs improvement”, 6% “poor”. 101% total, once again Bezos gets more than he deserves or needs.

Here’s what those numbers really mean. Over the previous 28-day collection period, this page has had:

  • LCP lower than 2.5 seconds in 87% of loads
  • LCP between 2.5 and 4.0 seconds in 8% of loads
  • LCP higher than 4.0 seconds in 6% of loads

The page loads happen on many different real-world devices and network connections.

And these page loads, the data you see comes from the CrUX report. CrUX (Chrome User Experience) report collects anonymized metrics from real users visiting your URL. This is called field data – it is collected in the field. As long as a URL has enough hits, the data for it will be available in CrUX, even the website was not submitted to Google Search Console. Google likes collecting data as much as a dog likes a bone.

There are two ways to look at the Field Data from the screenshot above:

  • Nailed it, it’s all green, time to celebrate and not worry about this again
  • Wow, 14% of all page loads for this URL have a sub-par experience?

What’s your approach, Jeff?

Lab Data

If you still want to improve your Field Data values, the way to do measure the effect of the changes you make is to check your Lab Data scores. Doesn’t make too much sense to wait for 28 days to see if your Field Data numbers went up or down.

But what exactly does Lab Data represent, and why is it so bad on mobile? Lab Data is collected differently than Field Data, it is recorded using Lighthouse (webpage assessment tool), from a single page load which happens under specific circumstances:

  • Latency: 150ms
  • Throughput: 1.6Mbps down / 750 Kbps up
  • 4x CPU slowdown

Or if you’re a more visual person, something like this:

Moto G4 is the device used for mobile assessment in Lighthouse
Moto G4, the reason your Lab Data numbers are low

That’s a 2016 Moto G4, Lighthouse’s device of choice. On a slow connection. For most websites, this is probably a worst-case scenario when it comes to device/network combination, but as you saw from Amazon example, it is still possible that a good share of your users will be on a device/network combination that you may think of as unrealistically slow.

This is why you should still work on those Lab Data scores, even if your Field Data looks great. Your brand new iPhone on a 1 Gb network is not how everyone will experience the website. You have a choice here – either make the website load well for everyone, or live with the fact that you’re leaving some people out. You flying on a spaceship, while others have to wait 5 seconds before anything shows up on their phone screens is not cool, right, Jeff?

And don’t forget, if you are looking for help fixing your Core Web Vitals scores, we can help you with that!

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