Study design
A 2 (branded packaging, unbranded packaging) × 3 (no warning label, text warning label, graphic warning label) × 2 (no tax, 20 % tax) between-group experimental study design was used, with the intervention scenarios delivered online and outcome data collected entirely via online survey. The study was undertaken in New Zealand over one week in August 2014. Ethical approval was received from the University of Auckland Human Participants Ethics Committee on August 1st 2014 (reference number 012200) and all study participants provided informed consent.
Recruitment
People were eligible for inclusion in the study if they reported consuming SSB in the previous two months, and were aged between 13–24 years. Recruitment of 16–24 year olds was undertaken by a market research company, Research Now, which invited a random sample of their panel members aged 16 or older to participate in the study. Younger participants (aged 13–16 years) were additionally recruited via parents who were panel members. Recruitment continued until at least 600 eligible respondents consented to take part and were randomised. All participants received a nominal monetary reward (NZ$1, equivalent to US$0.72, €0.64 on 20 May 2016) for 16–24 year olds, and NZ$3, (US$2.17, €1.93) for parents who consented to their 13–16 year old children participating) on completion of the study.
Interventions
Participants were randomly allocated using a computer-generated algorithm to view one of 12 experimental conditions, specifically images of branded versus plain packaged SSB cans, with either no warning, a text warning, or a graphic warning, and price with or without a 20 % tax (Fig. 1). Images were of a well-known carbonated SSB, and the same brand name was used throughout. The plain packaging image used the same colour as that regulated under Australia’s tobacco plain packaging laws, ‘Pantone 448 C’. The brand name was written in size 14, bold Arial font.
The text warning was presented in an orange-coloured octagon and covered approximately one eighth of the front side of the can. It stated “WARNING: high sugar content” in size 6, bold Arial font. The format was based on that of the Chilean warning labels for foods high in energy, sodium, saturated fat or sugar to be implemented in July 2016 [22]. The graphic warning label was modelled on graphic tobacco health warnings currently used in New Zealand, and displayed an image of dental caries with the accompanying message “WARNING: consuming beverages with added sugar contributes to tooth decay”. Tooth decay was chosen as the health outcome because there is a strong relationship between sugar consumption and dental caries [23], and it is a more immediate consequence of SSB consumption than obesity. As with the text-only warning label, the pictorial label covered approximately one eighth of the soft drink can’s face. The graphic part of the warning made up half of the label and the text took up the other half (size 6, bold Arial font for the ‘WARNING’ and size 4, bold Arial font for the rest of the message).
The tax/no tax scenarios were operationalised by presenting the usual (average) price of a can of the displayed SSB (NZ$2.00, equivalent to US$1.44, €1.29) or the price of a can with 20 % tax applied (NZ$2.40, US$1.73, €1.55) directly below the images.
Data collection and outcome measures
Prior to the survey, data were collected on participants’ key demographic characteristics, usual consumption of common foods and beverages, and frequency of consumption of a range of beverages including carbonated drinks, energy drinks, milk, and juices. These also served as screening questions to ensure respondents were in the target age range and had consumed SSBs in the previous two months.
Following randomisation, participants viewed the allocated image, and used a seven-point rating scale to answer questions regarding attitudes and predicted product preferences. Specific semantic differential attitude statements assessed whether participants believed the displayed SSB was: expensive/cheap, unattractive/attractive, low quality/high quality, uncool/cool, unhealthy/healthy, and tasted bad or good. As adolescents and young adults often consume SSBs in public settings, and brands are used to communicate aspects of consumers’ identities [24], we also asked questions regarding their perceptions of a peer if they were drinking from the can displayed. Perceptions were measured using four semantic differential questions anchored by: boring/interesting, unpopular/popular, unfashionable/fashionable, and old/young.
The 11-point Juster Scale was used to measure the “in the moment” probability of purchasing the displayed drink if it were one of the options available at a convenience store, where 0 represented “no chance or almost no chance”, 5 represented “fairly good possibility”, and 10 represented “certain or practically certain” [9, 25]. Five-point Likert scales were used to measure participants’ attitudes towards proposed implementation of warning labels and taxes on SSBs. Two versions of the survey were used, one for the young adult group (17–24 years), and another, with simpler language, targeted to the adolescent group (13–16 years).
Sample size and analysis
Assuming a standard deviation of 1.0, power calculations indicated that a sample size of 600 participants (50 participants per experimental scenario) would provide 80 % power to detect a minimum one-point difference in purchase probability on the Juster scale.
Independent sample t-tests were used to assess differences in mean predicted preference scores as a result of packaging or taxes, and a one-way Analysis Of Variance (ANOVA) was used to assess differences in product preferences as a result of warning labels. A three-way ANOVA was conducted to determine whether the three experimental scenarios were associated with significant effects on participants’ likelihood to buy SSBs. Multivariate Analyses Of Variance (MANOVA) were undertaken to assess the relationships between the interventions.
