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Head to Town Square on a busy Saturday during the summer, and it can feel like there are 25,000 people roaming the streets of downtown, a density more akin to an urban center than a rural county.

Maybe you even go home and Google the Teton County population, which the U.S. Census Bureau put at 23,464 in 2019. It might then strike you as odd that every person in Teton County was at Town Square, but the concept of effective population explains the disparity between Census numbers.

“Effective population indicates the true, day-to-day number of people in Teton County, considering not only permanent residents, but also commuters, seasonal residents, seasonal workers, and visitors,” states the Teton County 2020 community indicator report.

The concept is crucial for public planning in a tourism-dependent town. Particularly in summer and winter, extra tourists, seasonal workers and second-home owners put strain on public transportation, St. John’s Health, roads and other infrastructure.

Without accounting for them, facilities and emergency responders could be overwhelmed. In recent weeks, several pieces of public comment sent to elected officials have asked for the effective population to be used when evaluating COVID-19’s impact on the community.

But it doesn’t work like that, public health officials warn.

“You use a rate per 100,000 using the Census on every single metric that’s health related,” Teton County Director of Health Jodie Pond said. “Whether that’s chlamydia rates, or diabetes, that’s the standard.”

The rate of new daily cases per 100,000 people has been a metric public health officials have raised during community updates. It’s a way of comparing towns of different sizes, so by using it, Jackson can be compared to other resort communities like Aspen, Colorado, or Sun Valley, Idaho, even though their populations aren’t exactly the same.

Some public commenters say Teton County’s effective population is a more accurate representation of how many people are in the community and would show the outbreak is less severe. The county indicator report puts Teton County’s 2018 summer effective population at 68,007, so using that instead of the Census number would decrease Teton County’s rates by nearly a factor of three.

During the most recent COVID-19 community update, Pond warned that switching to the effective population would effectively under count the severity of the pandemic. That’s because visitors, commuters and others passing through are typically counted in their home counties — not Teton County — when they test positive for the virus. So their numbers are generally not artificially inflating the local case rate.

“If we were counting all those folks in our numerator, then we could use the effective population in our denominator, but we don’t do that,” she said. “Those are not our cases, so it doesn’t make sense to change the denominator.”

The county’s effective population estimate also includes seasonal national park workers, not all of whom live in Jackson, and this year’s count is likely not reflective of previous years. More second-home owners might be here, but J-1 visa worker numbers have decreased because of travel restrictions and the pandemic.

“So what is the number?” Pond said. “Short of trying to do a survey to figure out how many seasonal workers we actually ended up with these three months, I don’t know what that number is.”

The focus on effective population, Pond said, misses one crucial aspect of public-health decision making. Rates of new cases, which the effective population would change, is just one of nine metrics officials track.

In addition, they evaluate these factors using a three-tiered system, ranking them each improved, stable or concerning. Rather than using case rates as a benchmark, officials track them over time, so the number itself is less important than whether those numbers are trending up or down over time.

“It’s a trend line that we’re looking at,” Pond said.

Contact Tom Hallberg at 732-7079 or thallberg@jhnewsandguide.com.

Tom Hallberg covers a little bit of everything, from skiing to long-form feature stories. A Teton Valley, Idaho, transplant by way of Portland and Bend, Oregon, he spends his time outside work writing fiction, splitboarding and climbing.

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