Angela Lang/CNET

If you've ever noticed a YouTube or Netflix video slowing down on your phone, even when you have excellent network coverage, your data might be throttled. A recent study by Northeastern University and the University of Massachusetts at Amherst found that AT&T, Sprint, T-Mobile and Verizon have all artificially slowed down online videos. If you're unsure of how to tell whether your phone carrier is slowing down your data, you can test it with the Wehe app.

Wehe's tests to show you the speed of certain apps, like Netflix and YouTube, compared to the other apps running on your phone. A difference between the two typically indicates that your Internet service provider has slowed down the app in question.

We ran Wehe on all four major US carriers. The results indicated no differentiation for Spotify (meaning it doesn't appear to be throttled), but YouTube was slower on almost every carrier.

Screenshot by Katie Conner/CNET

Verizon: Spotify no differentiation, YouTube throughput 4.5 Mbps versus non-throughput 17.7 Mbps

Spotify no differentiation, YouTube throughput 4.5 Mbps versus non-throughput 17.7 Mbps T-Mobile: Spotify no differentiation, YouTube 1.5 Mbps versus 17 Mbps

Spotify no differentiation, YouTube 1.5 Mbps versus 17 Mbps AT&T: Spotify no differentiation, YouTube slight differentiation detected 1.2 Mbps versus 1.5 Mbps

Spotify no differentiation, YouTube slight differentiation detected 1.2 Mbps versus 1.5 Mbps Sprint: Spotify no differentiation, YouTube no differentiation

AT&T said it doesn't throttle, discriminate or degrade network performance based on content.

"We offer customers choice, including speeds and features to manage their data," AT&T spokesman Jim Greer said in an emailed statement. "This [Wehe] app fails to account for a user's choice of settings or plan that may affect speeds. We've previously been in contact with the app developers to discuss how they can improve their app's performance."

In a message, Choffnes said that the team doesn't deny that the plan a user chooses or the plan's settings can affect throttling.

"What is true is that AT&T throttles a significant fraction of our user's Wehe video tests (about 70%). So at least for our users, it seems very likely that their plan/settings are set up to throttle video," Choffnes said.

T-Mobile declined to comment on the study but pointed to a 2015 press release about its efforts to enable more streaming.

Verizon and Sprint didn't immediately respond to a request for comment.

Here's how to see if your videos are running slower than your other apps.

1. First, you'll need to download the Wehe app for Android or iPhone.

2. Make sure your Wi-Fi is turned off so that your phone will use your carrier's network during the tests.

3. Select each app you want WeHe to run a test on. Your options include YouTube, NBC Sports, Skype, Vimeo, Amazon Video, Netflix and Spotify.

4. Tap Run Tests. Give it a few minutes to load the test results.

5. If the results show there's no differentiation, then the app is running smoothly. If there is differentiation, that means your network could be throttling your data.

Did you test your apps on Wehe? Let us know your results in the comments. If you want to know more about your phone's speed, check out (article).

Need a speed boost? Here's how to boost your phone's slow data connection or poor signal strength.