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Evaluation 2 of "Do Celebrity Endorsements Matter? A Twitter Experiment Promoting Vaccination In Indonesia"

Evaluation of "Do Celebrity Endorsements Matter? A Twitter Experiment Promoting Vaccination In Indonesia" by Anirudh Tagat for the Unjournal

Published onAug 25, 2023
Evaluation 2 of "Do Celebrity Endorsements Matter? A Twitter Experiment Promoting Vaccination In Indonesia"

Summary Measures

Overall assessment

Answer: 85

Confidence (from 0 - 5): 4

Quality scale rating

“On a ‘scale of journals’, what ‘quality of journal’ should this be published in?: Note: 0= lowest/none, 5= highest/best”

Answer: 4

Confidence (from 0 - 5): 5

See the evaluation summary for a more detailed breakdown of the evaluators’ ratings and predictions.

Written report

Note from David Reinstein, Evaluation Manager: This evaluator considered the May 2023 (MIT) working paper version titled “Do Celebrity Endorsements Matter: A Twitter Experiment Promoting Vaccination In Indonesia

Brief explanation:

  1. This paper is important for understanding how celebrities and in general, influencers could play a role in health communication in a developing country context (Indonesia). It provides a novel and carefully designed experiment where celebrities post messages on Twitter that are intended to boost immunization rates and awareness in the country.

  2. It is grounded in a rigorous theory drawing from network science as well as the economics of networks, and makes a clear contribution to newer applications of network theory in the realm of applied economics work.

  3. The empirical framework aims to cover various aspects of how the intervention that the authors implemented might operate, focusing in detail on potential mechanisms (as explained by their theoretical framework), as well as on two main questions: (a) what characteristics of a message have the most reach; and (b) whether seeing these messages has some downstream impact on knowledge and beliefs around immunization.

  4. The paper is well written and clearly a very rigorously conducted experiment that has tremendous value to those working in health policy and health communication. In terms of causal inference, however, there are a few concerns that the authors could take into account to strengthen the confidence in their results, especially those surrounding the impact of these messages on knowledge and beliefs (“offline sample”).


This paper looks at the impact of celebrity endorsements on Twitter related to immunization in Indonesia. The authors recruited 46 celebrities and 1032 ordinary citizens to participate in their study, and randomly assigned each of them to tweet a message around immunization practice during the July 2015 to February 2016. This allows authors to exploit randomized variation in the messages to study important and relatively unexplored aspects of how messages are passed on within large networks (especially networks formed on social media platforms such as Twitter). They vary the timing of the tweet, who it is originally composed by (an ordinary citizen or a celebrity), and whether or not the tweet had a source attached. This type of randomization is important because it allows one to disentangle the endorsement effect from the reach effect, an important contribution of this paper. The endorsement effect refers to the response to a celebrity directly tweeting a particular message, whereas the reach effect refers to the response when the celebrity simply re-tweets the message that was posted by an ordinary citizen. This effect is important to disentangle using careful randomization since doing so using observational data/econometric methods is challenging. Given that the authors had an opportunity to randomize, they are able to distinguish this. From a methodological perspective, this is a significant value addition to emerging work that uses social media data to study a range of issues including polarization, misinformation, and message diffusion (the topic of this paper).

The paper has some important findings:

  • Messages that come (originally) from celebrities are more likely to be engaged with (retweeted or liked), relative to those that come from ordinary citizens. This finding is important to understand how celebrities (and their endorsements) are central to maximizing the reach and engagement of public health messaging via social media.

  • The second finding is that including a source in the message (which the authors refer to in their design as the credible sourcing treatment) actually reduces the likelihood of engaging with a message. This is counterintuitive (as the authors admit), since adding an information source should ideally boost the likelihood of engagement, not reduce it. They do well to use their theoretical framework to explain that this might be on account of the lack or originality associated with a message that comes with a source. By introducing this variation randomly, the authors are better able to build on the value of celebrity endorsements, by also being able to study what type of message is likely to get the most reach and engagement when it comes to health communication in Indonesia.

  • Last, they find that being exposed to these messages increases vaccine-related knowledge, promotes accurate and scientific beliefs around immunisation, and has better recall (i.e., that participants are more likely to recall the associated hashtag). This last piece of evidence is actually the most critical to public health, since it suggests that such campaigns can have measurable real-world impacts, outside of the social media platform on which such interventions can be designed and implemented.

Here, I restrict my comments primarily to the causal inference aspects of the paper, and potentially ruling out other explanations for the results.

  • Overall, given the count outcomes data used, the Poisson regression model is a reasonable choice for estimation framework, and they also use a rigorous method to control for multiple hypothesis testing. However, they would do well to attempt to use existing methods that take into account a similar approach (e.g., Young 2018).

  • First, in modelling the user response to a tweet (whether it comes from a celebrity or another ordinary citizen), the authors acknowledge the potential endogeneity arising from how much the message has been retweeted or liked at the time that the user saw the message. It is of course challenging to do this, but using timestamps, there could potentially be a way to account for the immediately preceding reach or engagement that the tweet received as this could influence to a large extent whether a user chooses to like or retweet the message. They attempt an exercise similar to this in Table E.1 (in Appendix E of the 2023 working papers), but it proceeds with arbitrary linear thresholds (i.e., 5, 10, 15 tweets), whereas it should ideally enter as a continuous variable in this estimation.

  • It also could help link this work to other work in computer science and networks on how messages go viral (Berger and Milkman 2013), which suggests that tweet engagement is likely to be a function of emotions and may not always follow a clear chain as described in the paper. These may not be mutually exclusive per se, but I suspect that incorporating emotions in the model may be challenging as they can perhaps only be primed in the messages (e.g., an emotional appeal could be another treatment group that was randomly varied).

  • Second, tweet engagement is also strongly determined by the Twitter algorithm, which we have no clear idea of. This means that the fact that a user sees or engages with a tweet is not solely due to the fact that a certain number of people tweeted it, but also how the algorithm weights the importance of that tweet to audiences that may or may not have mutual network nodes with those sharing the content. The authors make a mention of how the feed works and also note changes in the algorithm in a period around their study (footnote 10), but it remains unclear if the implemented randomization translates uniformly to all Twitter users in Indonesia. This brings me to the main issue with the suggestive evidence provided on impacts of this intervention on the “offline” sample using phone surveys. The authors acknowledge the limitations of this, but I think they need to also emphasize that these are mainly correlations and cannot trace any causality back to their interventions, which is an important limitation of this study, given that the offline data relies on self-report information related to knowledge and beliefs around vaccination.

  • One way to overcome this is to perhaps use secondary data (if available) on child immunization per capita correlated with the regions where the hashtag #AyoImmunisasi received the most engagement. This data may not be public, but given access to the API, it may be possible to correlate the two.

  • Translating online engagement to offline behaviour is without a doubt challenging, and a very ambitious ask for a project that already has quite a bit of novelty and rigor in terms of experimental design and inference. However, it helps to go beyond self-report data, and also correlate with whether such knowledge and beliefs might have changed owing to the large-scale social media campaign (for which panel data from users before and after the Twitter intervention would be needed).

Two other minor things to consider in terms of the validity of the design:

  1. Can users (including celebrities) delete the tweet once it is sent out as part of their participation in the experiment? This would also affect the extent to which it has reach or engagement. If authors could report on whether this was possible (which I suspect it was, although recruited individuals may have had to agree to not delete the content after it was tweeted).

  2. Finally, in an older version of the paper (NBER working paper, 2019), estimation of equation 3.3 was conditioned on a sample of individuals who followed at least 3 celebrities. It was not clear how this criteria was developed. I could not find this in the most recently updated version (2023).

Overall, this paper offers important and critical information on effectiveness of public health messaging via social media. It also helps to disentangle important aspects of reach vs. engagement in social media, when those with power on these platforms send out messages, and precisely the channel through which one might expect diffusion. The offline sample results, although important from a public health outcomes perspective, are correlational in nature, and can be ideally put down as a supplementary part of the paper or bolstered with some secondary data on immunizations before and after the intervention.

Evaluator details

  1. How long have you been in this field?

    • I am 2 years post-doc, and have been working in applied microeconomics in a developing country context (India) since 2014, and have been working on health policy and communication (with a focus on behavioural science) since 2020.

  2. How many proposals and papers have you evaluated?

    • For the Unjournal, I have been managing editor on one other evaluation, and have evaluated one other paper. As a peer reviewer, I have done about 25 papers, largely in the domain of health and applied economics. I am also currently Deputy Editor at South Asia Research, where I edit papers in economics.

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