SLOTS polling of voter intent conducted by the Institute for Health Policy (IHP) since the Presidential Election through 8 November 2024 has suffered from an increased level of response bias. This has been due to respondents over-reporting support for the NPP. This has resulted in large uncertainty and a significant bias in favour of the NPP in the resulting vote intent estimates.

Nonetheless, we are releasing estimates based on these polling data in the interests of transparency. We advise that these estimates are likely to be associated with a large margin of error, and caution should be taken when interpreting them. As with any opinion poll, they should not be regarded as a prediction of the final election result.

Starting in about June 2024, the IHP SLOTS poll experienced increasing bias in survey responses. The share of respondents claiming to have voted Anura Dissanayake in 2019 increased from 2–3% in the previous two years of polling to 10–15%, compared with the actual election result of 3%. Since the Presidential Election on 21 September, this trend has been reinforced, with over 60% of respondents claiming they voted for Anura Dissanayake in this year’s election, and additionally claiming intent to vote for the NPP at the forthcoming General Election. These numbers are inconsistent with the actual election results and point to substantial bias in our polling due to significant social desirability bias favouring the NPP. This bias is likely to result in over-estimate of support for the NPP. Although IHP takes steps to counter bias in its analysis, it is not possible to fully do so as the level of bias in this case is unknown.

Our best analysis of the SLOTS survey data estimates support at end-October/early November for the NPP at 53% of all adults, followed by the SJB with 26%, NDF with 9%, SLPP with 7%, and the ITAK with 2%. These figures are subject to considerable uncertainty and a subjectively assessed margin of error of 5–7%. This error may be even larger for the reasons mentioned above. Internal modelling of these estimates indicates this would translate into 120–130 seats for the NPP, followed by the SJB with 55–65 seats.

Because of the bias in our data, we cannot exclude the possibility that NPP support may be higher than implied above. In that scenario, the NPP may win a two-thirds majority in Parliament, but we have less confidence in this outcome.

IHP’s SLOTS MRP methodology first estimates the relationship between a wide variety of characteristics about respondents and their opinions – in this case, ‘If there was a General Election today, who would you vote for?’– in a multilevel statistical model that also smooths month to month changes. It then uses a large data file calibrated to the national population to predict voting intent in each month since Oct. 2021, based on what the multilevel model indicates about their probability of voting for various parties (‘post-stratification’) at each point in time.

IHP’s data collection covers all districts and communities, and it uses raking to ensure that the estimation samples fairly represent the adult population with respect to age, gender, ethnicity, religion, province and district, sector of residence, relative socioeconomic status, and education.

About IHP

IHP is solely responsible for commissioning and designing the survey, and it takes full responsibility for it. IHP is an independent, non-partisan research institution based in Colombo, Sri Lanka. The SLOTS lead investigator is Dr Ravi Rannan-Eliya of IHP, who was trained in public opinion polling at Harvard University, and who has conducted many opinion surveys over three decades, both in and outside Sri Lanka.

Methodology

SLOTS combines interviews from a national sample of adults (ages 18 and over) reached by random digit dialling of mobile numbers, and others coming from a national panel of respondents who were previously recruited through random selection. IHP estimates voting intent using an adaptation of Multilevel Regression and Post-Stratification (MRP), with multiple imputation to account for uncertainties in its modelling, exploiting data from all SLOTS interviews to estimate voting in a particular month.

Funding

IHP conducts the SLOTS survey to track changes in health and social conditions, and public opinion in the country. IHP is solely responsible for conceiving, commissioning and designing the survey, and it takes full responsibility for it. Interviews are done daily by phone by IHP employees, with respondents recruited by a national field survey or by randomly dialling phone numbers. SLOTS fieldwork since 2021 has been supported by a range of funders, who play no role in question design, data analysis, or reporting. Past funders have included the Neelan Tiruchelvam Trust, Asia Foundation, European Commission, UK National Institute for Health and Care Research, the Foundation Open Society Institute, and others. Current fieldwork is supported by funding from the Velux Stiftung foundation, New York University Abu Dhabi, USAID, and the IHP Public Interest Research Fund. The survey findings and IHP reporting do not necessarily reflect the views or positions of past and present funders. Interested parties can contact IHP for more detailed data and results.