Table 1 compares the characteristics and question sets of the surveys examined in this study. Looking at the table, similarities are apparent not only in the sets of questions on services utilization, but also in the individual questions regarding potential determinants of utilization. There are, however, noticeable differences in the objectives of the surveys in terms of study population and design. For example, whereas the RKI surveys have a one-time cross-sectional design, the SOEP and Bertelsmann Health Monitor involve repeated panel surveys. In addition, the SOEP contains noticeably fewer questions on subjective health and on disabilities and limitations in daily living activities.
Survey characteristics and question sets
Looking at Table 2 , clear differences can be seen in the surveys’ questions on the utilization of outpatient services. Only three out of six surveys ask participants when they last contacted a physician, and only the BGS1998 and the Health Monitor surveys ask about the reason for the last contact or for more details about the contact. In surveys containing questions on contact with a doctor during a specific time period, these periods range from four weeks to twelve months. There are also major differences in the questions regarding contacts with specific types of physicians. Whereas some of the surveys do not ask about contacts with specific physician groups at all, the types of physicians inquired about in the other surveys vary. Also, only a few surveys contain questions on the use of screening and prevention services.
Questions pertaining to utilization of outpatient services
The questions on outpatient utilization also differ in their wording with possible implications for the answer, e.g., “What kind of specialist did you demand and how often in the last 12 months?” (BGS98) or “How often do you visit a primary physician in the last 12 months?” (GEDA09) or “To what kind of specialist did you have contact in the last 12 months and how often?” It is unclear for the participants whether only direct contacts were meant or also practice consultations, e.g., for a drug prescription.
Greater similarities can be found in the surveys’ questions on the prevalence of hospitalization. All six studies asked whether there had been an inpatient stay during the previous twelve months. The only differences here were the way the surveys inquired into the frequency of inpatient care (i.e., the number of nights spent in a hospital or number of hospitalizations over the previous twelve months; see Table 3 ). Questions regarding the utilization of other services (rehabilitation services, non-physician services, the use of prescribed or over-the-counter medications) will not be discussed here as they are not standard questions in the surveys analyzed here.
Questions regarding the utilization of other medical services
The East-West Survey (OW1991), the Federal Health Survey (BGS1998) and GEDA2009 survey all ask participants when the last time was that they had contacted a physician. The results of these surveys regarding whether a physician had been contacted within the previous four weeks differed by approximately 10 percentage points, with participant percentages varying from approximately 40 to 50 percent. Extending the reference period to 3 months, the same percentage point difference can be observed (63–72%). Age and sex-specific patterns of outpatient care utilization are also consistent; again, the surveys’ results differ by approximately 10 percentage points. Unlike the other surveys, the Bertelsmann Health Monitor does not provide any data on age or sex-specific use of outpatient care. It only inquires into the number of outpatient contacts made with different physicians over the previous twelve months (Table 4 ).
Point in time of last outpatient contact, by age group and sex
The four-week prevalence of contact with GPs was 29% in the OW1991. According to the BGS1998, the twelve-month prevalence was 71%. Whereas the OW1991 reported an average of 1.8 contacts with GPs or primary care physicians over the previous 4 weeks, the BGS98 and the Bertelsmann Health Monitor each reported an average of 4.9 contacts over the previous 12 months and the 2003 Telephone Health Survey reported 4.0 contacts, though only for survey participants indicating that they have a regular primary care physician. There is marked variation in the reported number of physician contacts made by survey participants. The GEDA2009 and SOEP surveys only asked about the number of contacts made with physicians in general. The results were an average of 6.1 physician contacts made over a twelve-month period (GEDA2009) and an average of 3.6 contacts made over a three-month period (SOEP). While the Bertelsmann Health Monitor distinguishes between contacts made with four different types of physicians, the OW1991 and the BGS1998 ask about contacts made with over ten different types of physicians when examining the frequency of services use (Table 5 ).
Prevalence and frequency of outpatient contacts with different types of physicians, by age group and sex
Even for identical questions on whether participants had stayed in a hospital over the previous twelve months, there were varying results, which ranged from around 9 to 15%. This variation can only partially be attributed to the different surveys populations since there are still noticeable differences even after stratification by age group. The patterns of age- and sex-specific inpatient care use are, however, the same in all surveys (Table 6 ).
Prevalence of inpatient care in the last 12 months, by age group and sex
Due to inconsistency in the questions asked in the RKI surveys, these surveys are less suitable for showing changes utilization of outpatient care over time. The Bertelsmann Health Monitor and the Socioeconomic Panel, however, study the same questions over a longer time period. Nearly 95% of participants in the Bertelsmann Health Monitor reported having contacted their primary care physician at least once over the previous year, with an average total of 5 contacts during that period of time (Figure 1 ). The proportion of participants reporting at least one contact in the last 12 months is nearly constant over the 16 waves, also the mean number of contacts to their primary care physician and the interquartil range of this item. In every wave about ten percent of the participants reported more than ten contacts with maxima between 50 and 100 contacts.
Prevalence of primary care physician visits in the last 12 months according to the 16 waves of the Bertelsmann Health Monitor
According to the results of the SOEP regarding inpatient hospital stays over the previous 12 months, the percentage of survey participants with at least one hospital stay remained a steady 11–12%. There was also a steady decline in the average number of overnight hospital stays among the group of participants who reported at least one hospital stay (Figure 2 ). As with outpatient contacts, a small proportion of participants (≤10 percent) reported more than 30 night spent in hospital in the last 12 months, probably due to severe acute diseases or chronic and/or psychiatric illness.
Prevalence of inpatient admissions and number of nights spent in hospital in the last 12 months according to the 16 waves of the SOEP