The 2025 annual survey of the panel Health in Germany by the Robert Koch Institute: Background, Methodology, Panel Attrition, and the integration of the European Health Interview Survey (EHIS)

Tracking the health of a nation is like trying to keep a perfect scorecard for a game that never stops. People move, change their minds, get sick, or simply decide they no longer want to be part of the count. In public health, researchers rely on surveys to understand how well a population is doing, but these snapshots can become blurry if the people answering the questions stop showing up or if the group answering changes in ways that no longer represent the whole. This is the central challenge of panel studies, where the same group of people is asked to answer questions repeatedly over many years. The goal is to see how individual lives change over time, but the process is fragile. If the people who drop out are different from those who stay—perhaps because they are sicker, poorer, or busier—the final picture of the nation's health becomes distorted. To keep this picture clear, scientists must constantly monitor who is leaving, why they are leaving, and whether the remaining group still looks like the country they are meant to represent.

In 2025, the Robert Koch Institute in Germany published a detailed report on the second year of its "Health in Germany" panel, a massive, ongoing study designed to track the health of the German population. The researchers faced a year of significant changes, including the integration of a major European health survey and adjustments to how they rewarded participants for their time. Their work focused on three main questions: how many people stayed in the study, whether the group that remained still looked like the general population, and if the health data collected was still reliable despite people dropping out. The study revealed that while the number of participants shrank over the year, the core group remained surprisingly stable and representative. However, the researchers found a specific warning sign: people with lower levels of education were becoming harder to reach, a trend that required careful statistical correction to ensure the final health numbers were accurate.

The study began with a large group of people recruited in 2024, but by 2025, the researchers needed to refresh the panel to include younger people and fix gaps in the original sampling. They added new groups of participants, including teenagers aged 15 to 17, who were needed to meet the requirements of the European Health Interview Survey, a standardized survey used across the European Union to compare health data between countries. This integration meant the German panel had to align its questions and methods with a strict European framework. The survey itself was conducted in four waves throughout the year, with participants answering different sets of questions in a rotating pattern. Some answered online, while others filled out paper questionnaires by mail. To encourage participation, the researchers offered incentives, but in 2025, they were forced to change the reward system. Instead of giving cash to those who filled out paper surveys, they switched to vouchers. This seemingly small change turned out to be a critical variable in the study's results.

As the year progressed, the researchers tracked who stayed and who left. They distinguished between two types of leaving. The first was active attrition, where a person explicitly decided to quit the study, often by withdrawing their consent or reporting that they had moved away or passed away. This type of departure was relatively low, affecting about one percent of the group between each survey wave. The second type was passive attrition, which occurred when the researchers removed people who had simply stopped answering surveys for a long time. This was a much larger group, accounting for nearly 15 percent of the panel. The researchers found that those who left actively were often older or preferred the paper survey method, while those who were removed for inactivity tended to be younger. Despite these differences, the combined effect of both types of leaving did not drastically alter the overall makeup of the group. The remaining participants still closely resembled the German population in terms of age, gender, and where they lived.

However, one specific group began to drift away from the national average. Participants with lower levels of education became increasingly underrepresented as the year went on. This shift was closely linked to the change in incentives. When the paper survey participants lost their cash rewards and received vouchers instead, their response rates dropped significantly. Since people with lower education levels were more likely to use the paper survey, this group disappeared from the data faster than others. The researchers also noticed that the health characteristics of the group remained largely stable, with only minor changes in things like self-reported health or physical activity levels. When they applied statistical adjustments to account for the people who had left, the health data looked even more consistent. This suggested that while the group was changing slightly, the core health trends were not being lost, provided the researchers corrected for the missing voices.

The study also highlighted the importance of the survey's design in capturing a true picture of the nation. By using a mix of online and paper methods and drawing participants from official population registers, the panel managed to reach people who are often missed by telephone surveys, such as the very young, the very old, and those with less education. The integration of the European survey allowed Germany to compare its health data directly with other countries, but it also introduced methodological challenges. The researchers noted that because the survey mode and sampling methods have changed over the years in Europe, comparing data from different years requires caution. For instance, comparing the 2025 results directly with a telephone survey from a few years prior might show differences that are due to the method of asking, not actual changes in health.

Looking ahead, the researchers plan to address the issues they identified. They intend to bring back cash incentives for paper survey participants to see if that brings back the people with lower education levels who drifted away. They also plan to try reaching out to the people who were removed for inactivity to see if they can be re-engaged, which would help reduce the loss of data over time. A new group of participants will be recruited in the coming years to replace those who have left and to test if offering surveys in multiple languages can improve participation among people with a migration background. The 2025 report serves as a vital check-up on the health of the study itself, confirming that while the panel is maturing and facing natural challenges, it remains a robust tool for understanding the health of Germany. The key takeaway is that maintaining a representative group requires constant vigilance, careful adjustments, and a willingness to adapt methods to keep the data honest and useful for the future.