The Biggest Workplace Wellness Trial Ever Run Found Almost Nothing. Here's What It Did Find.

The largest randomized controlled trial ever run on workplace wellness programs found that employees who were offered one reported doing more exercise and managing their weight more actively than employees who weren’t. After a year and a half, though, it showed no statistically significant difference in blood pressure, cholesterol, blood sugar, healthcare spending, or how often they showed up to work.

Both of those findings are true at once, from the same study, about the same program. The gap between them is more informative than either number alone.

The study

Katherine Baicker (University of Chicago) and Zirui Song (Harvard Medical School) published the results in JAMA in 2019, following a clustered randomized trial run at 160 worksites of BJ’s Wholesale Club between January 2015 and June 2016. Nearly 33,000 employees were involved. Nineteen worksites’ employees were offered the wellness program, the rest served as a control group that wasn’t, and eligibility for the program was determined by which location an employee happened to work at, not by whether they signed up. That randomization matters more than it sounds like it should.

Almost every wellness-program study that came before it compared people who chose to participate against people who didn’t. That comparison is contaminated from the start: people who opt into a gym-membership perk or a step-count challenge are, on average, already more health-conscious than people who skip it, so a chunk of any measured benefit is really just measuring who signed up, not what the program did to them. A randomized design sidesteps that problem by assigning the opportunity independent of anyone’s existing motivation, which is also why RCTs in this space tend to find smaller effects than the observational research that came before them: the older studies were measuring self-selection dressed up as a program effect, not a weaker version of the same finding.

What moved, and what didn’t

After 18 months, employees offered the program self-reported higher rates of regular exercise and actively managing their weight compared to the control group. That’s a real result, not nothing. It suggests the program succeeded at the thing closest to its own front door: getting people to say they were doing more, the same self-reported-engagement metric that leaderboard-driven wellness platforms lean on heavily when selling employers on renewal.

Nothing else moved by a statistically meaningful amount. No significant effect on clinical measures: blood pressure, cholesterol, glucose. No significant effect on total healthcare spending or utilization. No significant effect on absenteeism or how long employees stayed at the company. The self-reported behavior change didn’t visibly carry through to the harder outcomes that wellness programs are typically funded to produce, at least not within the trial’s 18-month window.

A follow-up analysis extending the same trial out to three years reported a similar pattern: durable increases in some self-reported behaviors, without a corresponding shift in the harder clinical and financial outcomes employers actually budget against.

Why the gap is the finding

It would be easy to read this as “wellness programs don’t work” or “wellness programs do work,” depending on which number you quote, and both readings miss what the study is actually showing: self-report and hard outcomes aren’t the same measurement, and treating them as interchangeable is where a lot of workplace wellness marketing goes wrong. A program can change what people say they’re doing, and even change what they’re actually doing within the limits of self-report accuracy, without that behavior change adding up to a measurable clinical or financial result in a year and a half. Health outcomes like blood pressure and healthcare spending respond to sustained behavior over years, not a single 18-month window, and self-report tends to inflate in the direction of “trying,” whether or not the trying fully sticks. The same slippage between what people report and what a plain log shows turns up outside healthcare entirely — a first-person 30-day meeting log kept across a nine-time-zone team found a comparable gap between how draining a week felt and what the raw hours actually added up to.

Calling that “the program failed” overstates what the data supports. An 18-month RCT is a reasonable tool for detecting a shift in what people report doing, and a poor tool for detecting the years-long effects that shift is supposed to eventually produce, especially at the modest effect sizes most real-world interventions achieve. How companies that treat sleep like a business metric have fared with a similar approach runs into a related version of this same measurement gap: the intervention changes what gets reported well before it changes what gets diagnosed.

What a better trial would need

The obvious follow-up question, whether a longer trial would have found the clinical effects the 18-month window missed, has a partial answer already. The three-year extension of the same BJ’s trial had more time to let any later effect surface, and still reported the same basic pattern: durable self-reported behavior change, no clear movement in the harder clinical and financial numbers. That’s not proof no such effect exists at any time horizon. It’s evidence against the claim that an 18-month or even three-year window was simply too short to catch it. If the effect were large and reasonably fast-arriving, three years of a nearly-33,000-person randomized sample is a large enough net to have caught at least a partial signal.

A more decisive trial would need either a larger effect size to detect (unlikely, given how modest most real-world wellness interventions are), a much larger sample to detect a small effect with confidence, or a longer horizon than almost any employer is willing to fund a randomized study across. In practice, that means the evidence is likely to stay roughly where Song and Baicker left it for a while: self-reported behavior responds to these programs, and the larger downstream outcomes they’re usually sold on remain unproven at the scale and cost employers are actually paying.

One trial, not a verdict on the category

This is one trial, at one retailer, with one wellness-program design, not a verdict on every corporate wellness initiative that exists. BJ’s program combined health-risk assessments, biometric screenings, and wellness activities in one bundle; a program built differently, with different incentives or a longer follow-up period, could plausibly produce a different result. A more defensible conclusion than “wellness works” or “wellness doesn’t work” is narrower and less quotable: the self-reported behavior change and the clinical outcome change are separate questions, and a program that clearly succeeds at one has not, by itself, demonstrated anything about the other. Anyone citing a program like this as proof of hard financial ROI is citing a number the trial didn’t produce.

Applied more broadly: the same measurement gap is worth checking on any behavior-change product, DontSnooze included. A self-reported or engagement-based metric moving in the right direction is a different fact from an outcome further downstream actually changing, and it’s worth asking which one a given claim is measuring before taking it as proof of the other.

FAQ

Do workplace wellness programs improve employee health?

Not according to the largest randomized trial on the question. It found no statistically significant improvement in clinical health measures (blood pressure, cholesterol, blood sugar) after 18 months, despite a real increase in self-reported healthy behavior over the same period.

Do workplace wellness programs save employers money?

The same trial found no significant effect on healthcare spending or utilization, which runs against a common marketing claim that these programs pay for themselves through reduced medical costs.

Is this the final word on whether wellness programs work?

No single trial should be treated as the final word on an entire category. It’s the largest and most rigorously randomized study on the question to date, and its pattern (self-reported behavior improved, hard outcomes didn’t move within either an 18-month or a three-year window) is worth taking seriously as a caution against assuming the two always travel together.

Song Z, Baicker K. “Effect of a Workplace Wellness Program on Employee Health and Economic Outcomes: A Randomized Clinical Trial.” JAMA. 2019;321(15):1491–1501.