Table 2.
Store-Level Difference-in-Differences in Beverage Volume Sales 2 Years After Philadelphia Beverage Tax Implementation
| Philadelphia vs Baltimore | PA border vs MD border | |||||
|---|---|---|---|---|---|---|
| Outcome | Mean (SD) pre-tax volume sales (ounces, millions) in Philadelphiaa | Difference-in-differences (95% CI)b | Adjusted p-valuec | Mean (SD) pre-tax volume sales (ounces, millions) in PA bordera | Difference-in-differences (95% CI)b | Adjusted p-valuec |
| Taxed beverages | ||||||
| All | 1.03 (2.16) | −50% (−61%, −36%) | <0.0001 | 0.96 (1.45) | 16% (9%, 24%) | <0.0001 |
| Individual-sized | 0.13 (0.16) | −26% (−33%, −18%) | <0.0001 | 0.12 (0.14) | 13% (8%, 17%) | <0.0001 |
| Family-sized | 0.90 (2.01) | −53% (−64%, −39%) | <0.0001 | 0.85 (1.32) | 17% (9%, 25%) | <0.0001 |
| Sugar-sweetened | 0.88 (1.88) | −49% (−60%, −35%) | <0.0001 | 0.72 (1.10) | 17% (9%, 25%) | <0.0001 |
| Artificially-sweetenedd | 0.15 (0.32) | −55% (−63%, −44%) | <0.0001 | – | – | – |
| Nontaxed beverages | ||||||
| All | 1.29 (2.70) | 4% (−3%, 12%) | 0.29 | 1.17 (1.76) | 0% (−5%, 7%) | 0.90 |
| Individual-sized | 0.07 (0.11) | 1% (−4%, 6%) | 0.67 | 0.08 (0.12) | 2% (−2%, 6%) | 0.50 |
| Family-sized | 1.21 (2.60) | 4% (−3%, 13%) | 0.29 | 1.09 (1.65) | 0% (−6%, 7%) | 0.90 |
Note: Boldface indicates statistical significance (p<0.05).
Per 4-week period.
A generalized estimating equation with a log link and gamma distribution was used with indicator variables for intervention time (pre-intervention=0, post-intervention=1), city (Baltimore/MD border ZIP codes=0, Philadelphia/PA border ZIP codes=1), and an interaction between them, which represents the difference-in-differences. Percent change was calculated as (exp(difference-in-differences)-1)*100%.
P-values controlled for the false discovery rate for 8 comparisons in Philadelphia and 7 in PA border ZIP codes.
Difference-in-differences not reported for PA border vs MD border because of a violation of pre-tax parallel trends.