PUBH 4900 Week 6 Source Disagreement Memo Example

Memorandum

To: Capstone Instructor

From: BS in Public Health Capstone Student

Date: Month Day, Year

Re: How the capstone will weigh Friedman (2021) against Liu et al. (2023)

The Disagreement

Friedman (2021) reported that, against seven comparison districts, San Francisco's flavored tobacco sales restriction coincided with odds of recent smoking among high school minors more than twice as high. Liu et al. (2023) argued that the San Francisco data could not measure the policy at all, because the 2019 survey was fielded in fall 2018, before enforcement. Averaging these is impossible; one must be weighed above the other.

What Each Source Actually Claims

Table 1

The Two Sources Side by Side

Note. Assembled from the two publications.

Where They Really Disagree

They do not disagree about the numbers. Both accept that recent smoking among San Francisco minors in the 2019 wave was higher than in other districts. They disagree about what period those numbers describe. That narrows the dispute to a factual question about dates, which is far easier to settle than a methodological one.

Weighing the Design

On design alone, Friedman's analysis is stronger. It has a comparison group, prior trends and robustness checks, while Liu et al.'s Oakland figures are a simple before-and-after comparison with no comparison city. If both had measured the same period, I would give more weight to Friedman. The original analysis also made a sound choice in leaving vaping out, since the survey could not separate nicotine from cannabis, and that choice deserves credit even though the timing problem undoes the main estimate.

Weighing the Fact

But design cannot rescue a study whose exposure period is wrong. A difference-in-differences estimate compares change in a treated place with change elsewhere after treatment. If the "after" observation was collected before treatment, the estimate measures something other than the policy, perhaps chance in a small number of smokers or the effects of cannabis legalization that year, both of which Liu et al. (2023) raise. The original author's subsequent correction concerned exactly this point, the dates of survey waves (Friedman, 2022). Compliance data independently place the policy's bite after the survey: only 17% of retailers complied in December 2018 (Vyas et al., 2021).

Weighing the Interests

Liu et al. disclose that one author serves as a paid expert witness against tobacco companies. That is a reason to check their facts, not to dismiss them. Their central fact, the fielding window, came from CDC, and it is consistent with the compliance record produced separately by the city's health department. The disclosure therefore does not change my weighting.

Decision for the Capstone

The capstone will treat Friedman (2021) as a well-designed analysis applied to a period that, on the best available evidence, preceded the policy's enforcement. It will treat Liu et al. (2023) as decisive on timing and weak on effect. The verdict that follows is narrow: the original estimate cannot support a causal claim about the policy. It does not follow that the policy reduced youth smoking, which neither source shows for San Francisco.

What Would Reverse This

Evidence that a meaningful share of San Francisco's 2019 respondents answered after January 2019, or that compliance was much higher during fall 2018 than reported, would restore weight to the original estimate.

References

Friedman, A. S. (2021). A difference-in-differences analysis of youth smoking and a ban on sales of flavored tobacco products in San Francisco, California. JAMA Pediatrics, 175(8), 863-865. https://doi.org/10.1001/jamapediatrics.2021.0922

Friedman, A. S. (2022). Clarification and correction of survey wave collection dates in an analysis of youth smoking and a ban on sales of flavored tobacco products in San Francisco, California. JAMA Pediatrics, 176(9), 947. https://doi.org/10.1001/jamapediatrics.2022.2388

Liu, J., Hartman, L., Tan, A. S. L., & Winickoff, J. P. (2023). Youth tobacco use before and after flavoured tobacco sales restrictions in Oakland, California and San Francisco, California. Tobacco Control, 32(e1), e118-e120. https://doi.org/10.1136/tobaccocontrol-2021-057135

Vyas, P., Ling, P., Gordon, B., Callewaert, J., Dang, A., Smith, D., Chan, B., & Glantz, S. (2021). Compliance with San Francisco's flavoured tobacco sales prohibition. Tobacco Control, 30(2), 227-230. https://doi.org/10.1136/tobaccocontrol-2019-055549