We gave you wrong information. Here’s what happened.

This is a difficult post to write. But we have learned that a story we worked on this season contains factual errors. We’ve taken some time to find out what went wrong, to try to find out how it happened, and to think hard about what we can learn from it.

Here’s what we know.

Last summer, we embarked on an ambitious project: to test the accuracy of label claims of popular vitamins and supplements. We knew that there is very little independent testing done of supplements in Canada that is available to the public, so we wanted to find out if you’re really getting what you expect from the products you buy and trust.

So we did what we always do: We did research. We spoke with top experts in the field. And we decided to test a number of products to find out the answers for ourselves.

One of the experts we found, Neil Thanedar, is the co-founder of a U.S. company called LabDoor, which does just this kind of testing, analyzing and reviewing lab test results on supplements for the public. LabDoor tests hundreds of products a year; the company’s investors and partners include the Mayo Clinic and Rock Health.

Thanedar agreed to analyze our results. For our testing, he recommended a lab in Michigan, and Marketplace verified that the lab had the best credentials possible for this kind of testing: It has internationally recognized ISO accreditation, is registered with the U.S. Food and Drug Administration and is used by the vitamin and supplement industry for this kind of testing. We sent samples of popular products to the lab and waited for the results.

Thanedar wasn’t the only expert we talked to about the results. We spoke with several other experts, including Health Canada to make sure we understood what LabDoor had told us and double check that our conclusions were solid.

We also reached out to the companies and gave them the opportunity to respond to what we found. As often happens in our investigations, they disputed our findings and vigorously defended their products. Armed with both sides of the story, we felt confident about what we were reporting.

What we know now is that the lab got some of the results wrong. They didn’t just make a mistake on one test; we had several samples re-tested by other labs, and we now know that a number of results were incorrect. We still don’t know exactly how this happened, but the bottom line is one of the hardest things any journalist can ever have to face: Our report was wrong.

Here’s what we do know. We tested Emergen-C, a popular vitamin C product. The initial lab testing found that the product only contained one third of the amount of vitamin C the package promised. After re-testing samples from the same box at another independent lab, we now know there was no problem with the vitamin C levels in Emergen-C.

We also tested several protein powders for evidence of protein “spiking.” We know that spiking has been a problem in the supplements industry: It means that a manufacturer uses filler in its product because it’s cheaper or easier than the real thing.

Two of the products we tested, Cytosport’s Muscle Milk and GNC’s Lean Shake 25, appeared, in the initial lab testing, to be spiked. The GNC product appeared to have less than half the protein it promised. After retesting, we’ve discovered this is not the case: The products were not spiked.

We’re responsible for our journalism, and it’s a responsibility we take very seriously. The lab results and analysis were wrong, but we reported it. We also want to apologize to the companies in our report.

We also want to apologize to you, our viewers.

Every week, we ask you to trust that we’ve brought you investigations that matter to you and are true, fair and in the public interest. And for more than 40 years, we’ve worked hard to live up to that reputation.

We’ve learned from this experience: We’re taking a hard look at how we use labs. And we’re going to continue to push ourselves to bring you the kind of groundbreaking stories that you’ve come to expect from us. We have a lot of great stories in the works, and we hope that you’ll stay with us.