Washington State Institute for Public Policy

Soda taxes: a 1% higher tax on soda than on other food items
Public Health & Prevention: Population-level policies
Literature review updated November 2015.

Our review included two rigorous studies that investigated the relationship between a) taxes on soda that are greater than taxes on other food items, and b) obesity. One study examined the impact of soda taxes on children ages 3-18 and the other study examined the impact of soda taxes on adults at least 18 years of age. The effects presented here reflect a 1% increase in soda taxes beyond typical food tax rates.

META-ANALYSIS	CITATIONS

**Meta-analysis** is a statistical method to combine the results from separate studies on a program, policy, or topic in order to estimate its effect on an outcome. WSIPP systematically evaluates all credible evaluations we can locate on each topic. The **outcomes** measured are the types of program impacts that were measured in the research literature (for example, crime or educational attainment). **Treatment N** represents the total number of individuals or units in the treatment group across the included studies.

An **effect size** (ES) is a standard metric that summarizes the degree to which a program or policy affects a measured outcome. If the effect size is positive, the outcome increases. If the effect size is negative, the outcome decreases.

**Adjusted effect sizes** are used to calculate the benefits from our benefit cost model. WSIPP may adjust effect sizes based on methodological characteristics of the study. For example, we may adjust effect sizes when a study has a weak research design or when the program developer is involved in the research. The magnitude of these adjustments varies depending on the topic area.

WSIPP may also adjust the second ES measurement. Research shows the magnitude of some effect sizes decrease over time. For those effect sizes, we estimate outcome-based adjustments which we apply between the **first time ES is estimated** and the **second time ES is estimated**. We also report the **unadjusted effect size** to show the effect sizes before any adjustments have been made. More details about these adjustments can be found in our Technical Documentation.
Meta-Analysis of Program Effects
Outcomes measured	Primary or secondary participant	No. of effect sizes	Treatment N	Adjusted effect sizes (ES) and standard errors (SE) used in the benefit-cost analysis						Unadjusted effect size (random effects model)
				First time ES is estimated			Second time ES is estimated
				ES	SE	Age	ES	SE	Age	ES	p-value
Obesity	Primary	2	1365734	0.000	0.001	45	0.000	0.086	47	0.000	0.857

Citations Used in the Meta-Analysis

Fletcher, J., Frisvold, D., & Tefft, N. (2009). The Effects of Soft Drink Taxes on Child and Adolescent Consumption and Weight Outcomes, working paper.

Fletcher, J.M., Frisvold, D., & Tefft, N. (2010). Can soft drink taxes reduce population weight? Contemporary Economic Policy, 28(1), 23-35.