In the media, various agencies and columnists in Mexico have erroneously concluded that the Mexican tax on sugar-sweetened beverages (SSBs) did not reduce purchases of these products in 2015, as it did in 2014. This conclusion is supported by comparisons of aggregate sales data (raw data). In other words, this conclusion does not consider other factors that influence purchases and consumption of these beverages, that are independent of the effects of the tax—and that change from one period to another—. The purpose of this commentary is to explain the reason claims cannot be made about the effects of taxes in 2015 based on aggregate raw sales data. In order to isolate the effect of taxes on purchases or sales it is necessary to carry out statistical analyses that adjust for these factors.
To evaluate a public policy it is ideal to be able to randomly assign one group to receive the intervention and another group that does not. In this scenario, on average, the groups are almost equal in their characteristics and the only difference between them is that one receives the intervention and the other does not. Consequently, changes in the outcomes of interest between these two groups can be attributed with certainty to the intervention.
In contrast, the tax on SSBs was implemented in January 2014 throughout the country. An evaluation of this tax poses a challenge due to the fact that there is not an existing group of cities or regions where the tax on SSBs has not been implemented to make such comparison. Thus, when comparing changes in purchases or sales of SSBs before and after the implementation of the tax, the use of statistical methods is necessary to adjust for factors that change over time that may affect the consumption of SSBs, and that are independent of changes that result from the tax. This is critically important because if these variables are not adjusted for, erroneous conclusions can be made.
Therefore, a simple comparison of purchases or sales from one period to another (without adjusting for variables that change over time and that affect consumption) may lead to inaccurate outcomes. There are many factors that change over time that are associated with the demand and supply of beverages, in addition to prices, that increase with the tax.
These factors include the following:
- Changes in total population and/or age structure: an increase in total population increases the demand for beverages, and changes in age structure of the population may affect demand because some age groups in the population consume more sugar sweetened beverages than others. These changes occur from one year to another.
- Changes in economic activity from one period to another (growth or economic slowdown) can be associated with higher/lower employment or available resources, which in turn may affect the demand for SSBs.
- Seasonal changes: e.g., 2015 was a warmer year and temperature could have increased consumption of sugar-sweetened beverages as compared to previous periods.
- Previous trends: as observed, purchases/sales of SSBs experienced a slight decrease in the period prior to the tax, if a crude comparison of trends before and after the tax is carried out without taking this decline into account, the result could be mistakenly attributed to this decreasing trend rather than the tax.
Changes in marketing, promotions, new product offerings and other industry marketing strategies, which frequently include SSBs, can increase demand independent of the tax. It is known that promotions can increase consumption, due to their influence on unplanned purchases, which could lead to increases in consumption by up to 20%(1).
An additional element to consider in evaluating the tax is the comparison period. In order to evaluate the effect of the tax in 2015, the comparison must be made against the period prior to implementation of the tax, instead of comparing it against 2014 when the tax was already in place. This clarification is necessary since some of the comparisons being made are between 2014 and 2015.
Monthly Surveys of the Manufacturing Industry (EMIM, by its acronym in Spanish)
The EMIM provide monthly information about the manufacturing industry’s in-country product sales.(2) As shown in chart 1, the relative difference in average annual sales in liters of SSBs in 2014 (the year the tax was implemented) compared to the previous pre-tax period (2007-2013) is +6.4% and in 2015, the second year of tax implementation, it was +7.0%. When annual sales are compared taking population size into account (dividing sales in liters by total population for each year), the difference in 2014 is less then when a comparison is performed using liters: +1.6% in 2014 and +1.1% in 2015, both compared to the pre-tax period of 2007-2013 (Chart 2).
Finally, by using a statistical model and adjusting for other variables, such as seasonality, trends over time and the Indicator of Global Economic Activity, there was an average decrease of -6% in 2014 and -8% in 2015, as compared to the pre- tax period 2007-2013 (Chart 3). This example illustrates the importance of considering factors associated to the sale and consumption of beverages that are independent of the tax as part of this analysis. Without these adjustments, the (erroneous) conclusion would be that there was an increase in sales after the tax, both in 2014 and in 2015, while the adjusted analysis shows that there was a reduction in sales after the tax.
It is very important to note there are three studies on the effects of the tax on SSBs, each conducted independently, all of which show a reduction in the volume of purchases or sales of sugar sweetened beverages of 6% in 2014, using different models and databases: the analysis using the EMIM based on Chapa Cantú et al,(3) the study published by the National Institute of Public Health and the University of North Carolina,(4) and the unpublished report by the Autonomous Technological Institute of Mexico (ITAM, by its acronym in Spanish).(5)
Chart 1- Average annual sales of SSBs in liters before and after the tax
| Period of time | Amount |
|---|---|
|
2007-2013 |
18,237 millions of liters |
|
2014 |
19,404 millons of liters |
|
Relative Difference 2014 vs 2007-2013 |
+6.4% |
|
2015 |
19,523 millons of liters |
|
Relative Difference 2015 vs 2007-2013 |
+7.0% |
Based on EMIM data from 2007 to 2015. Relative Difference: [(annual sales post-tax - sales during pre-tax period) / sales during pre-tax period]]*100.
Chart 2-Annual Average Liters per capita of SSBs before and after tax
| Period of time | Amount |
|---|---|
|
2007-2013 |
160 liters /capita |
|
2014 |
162 liters/capita |
|
Relative difference 2014 vs 2007-2013 |
+1.6% |
|
2015 |
161 |
|
Relative difference 2015 vs 2007-2013 |
+1.1% |
Based on EMIM data from 2007-2015, and population projections (Consejo Nacional de Población). Relative Difference: [(annual sales post-tax - sales during pre- tax period)/sales during pre-tax period]*100.
Chart 3- Changes in liters per capita of sales of SSBs before and after tax (statistical model)
| Year | Change in percentage in regards to 2007-2013 |
|---|---|
|
2014 |
-6%* |
|
2015 |
-8%* |
Model adjusts for beverage trends over time (month count), seasonality (binary variables for each month of the year), Indicator of Global Economic Activity, *statistically significant at 1%) Chapa Cantu et al. Replication of model used by the study carried out by the Autonomous University of Nuevo Leon (UANL, for its acronym in Spanish).
Conclusions
The evaluation of the effects of SSB taxes in Mexico on purchases or sales after the tax require comparisons between the periods before and after the application of the tax, using data adjusted for variables that influence purchases and sales. Comparisons using raw data, such as those presented by various organizations, are not useful to evaluate the effectiveness of taxes.
References
- Public Health England. 2015. Sugar reduction: the evidence for action.
Available at: https://www.gov.uk/government/uploads/system/uploads/attachment_data/
file/470179/Sugar_reduction_The_evidence_for_action.pdf - INEGI. Banco de Información Económica (BIE). Manufacturas. Encuesta Mensual de la Industria Manufacturera. Available at: http://www.inegi.org.mx/sistemas/bie/
- Chapa Cantú, Flores Curiel D, Zuñiga Valero L. 2015. La industria de bebidas no alcohólicas. 2015. Universidad Autónoma de Nuevo Léon.
- Colchero, M. A., B. M. Popkin, J. A. Rivera and S. W. Ng (2016). "Beverage purchases from stores in Mexico under the excise tax on sugar sweetened beverages: observational study." BMJ 352: h6704. Available at: http://www.bmj.com/content/352/bmj.h6704
- Aguilar A. Gutiérrez E., Seira E. 2015. Taxing calories. Instituto Tecnológico Autónomo de México.
Spanish version: http://www.insp.mx/epppo/blog/4043-compra-venta-bebidas-azucaradas.html
