The number of longitudinal epidemiological studies on the period prevalence rates of headaches is limited. Our study is the first which determines the one-year prevalence of self-reported headache in representative samples of the German population over a long time period of 15 years by using the same methods (annual face to face interview). The most important result is that from 1995 to 2009 the one-year headache prevalence in the German general population is stable at 58.9 to 62.5%. The German National Telephone Health Interview Survey 2004 (GNTHIS), which accessed a large representative sample of the general adult population in Germany aged 18 years and older (n = 7,341), determined the one-year prevalence of headaches for the year 2004 as 60.3% (66.6% for women, 53.0% for men) . Our study showed prevalence rates of 62.5% (70.7% for women, 53.2% for men) in 2004 (Table 1). The German DMKG Headache Study with 7,417 interviewees, analyzing three cross-sectional surveys conducted in three German regions (Augsburg, Dortmund, Pomerania) between 2002 and 2004, found a sex- and age-stratified, pooled 6-month prevalence of 49.5% . The investigated regions in that study, however, are not representative of the general population in Germany . Another survey which used mailed questionnaires (4,061 interviewees) and a market research household panel, found a lifetime prevalence for headaches of 71.4% . In their meta-analysis “Global Burden of Headache”, Stovner et al. – based on 107 surveys – report 47% as the global period prevalence for “current headache”, which included 1 year, 3 months and ”time not stated”. When limited to surveys among adults, they report a prevalence of 46% . Thus, the one-year headache prevalence for Germany between 1995 and 2009 is about 10% to 15% higher. The reason for that is not clear, especially since the prevalence of migraine was slightly lower in the DMKG study than in comparable studies from other European countries [1, 4]. Only three other European surveys investigated the one-year prevalence and were conducted as face-to-face interviews, as in our survey. For Austria with 997 interviewees, the one-year prevalence was 49.4% (women 54.6%, men 43.6%) ; in Finland with 200 interviewees, it was 77% (women 83%, men 69%) ; and in Georgia with 1,145 interviewees, it was 46.3% .
An important result of our study was that the one-year prevalence of general headaches remains stable over a period of 15 years in Germany. This fact is quite remarkable since in this period dramatic political and economic changes took place in East Germany, where – beginning in 1990 – a socialistic economic system was transformed into a capitalist economic system. For the population of the former GDR this change in the economic and social system was equal to a loss of secure employment and confidence. It is obvious that such changes in lifestyle can have an influence on the stress experienced, which probably is one of the reasons for headache. Surprisingly we did not see such an effect in our survey which may indicate that the majority of the interviewees did not feel such a stress.
These stable prevalence rates are in agreement with other longitudinal studies from different countries. Four large cross-sectional surveys in the US also show stable one-year prevalence of migraine over a period of about ten years (1989 to 1998) [11, 12, 22–25]. Two methodically identical surveys conducted in France at an interval of ten years also showed a stable prevalence for migraine too [26, 27]. Also, the Norwegian HUNT 2 and HUNT 3 studies do not show any change in the one-year prevalence rates within eleven years [9, 28]. The results from the GNTHIS 2004 survey support our finding that there is no difference in the headache prevalence between East and West Germany either .
Another interesting observation in our study is that the prevalence rates were slightly, but noticeably lower in rural/small-town regions than in urban areas with over 50,000 inhabitants, which is in agreement with results from Austria . In contrast, the 1989 survey in the USA shows only minor regional differences – only mountainous areas, which are in general rural, showed higher one-year prevalence rates which is in contrast to our results. But here only interviewees with self-defined severe migraine were included and the regions were much larger than in Germany and Austria . The size of the place of residence had only a minor impact in this US survey with a slightly higher prevalence for severe migraine in the rural regions (< 50,000 inhabitants). A Spanish study also reported a slightly higher prevalence of general pain conditions (including headache) in urban areas compared to rural regions .
The question of whether household income and social situation have an influence on the headache prevalence rates is controversial. An English survey showed somewhat higher 3-month headache prevalence for interviewees from socially higher, non-manual working population strata than for manual workers . But this difference may be explained by differences in the age and sex distribution in the interviewed groups . In contrast a Spanish survey showed that the one-year prevalence of migraine was higher for subjects doing housework and unemployed subjects than for working people . But this study made no adjustment for the presumably higher percentage of women among those doing housework and the different age distributions. Although various surveys, in particular those from the US, associate the prevalence of headache diseases and especially of migraine with lower household income, the results are also contradictory. In the American Migraine Study II, migraine prevalence increased continuously with decreasing household income and was highest for men and women with an annual income below $15,000 . This confirmed results from 1989, which were gathered from a panel of 15,000 households  – for both migraine as well as all severe headaches. An analysis of the data of the National Health Interview Survey of 1989 also showed that self-reported migraine was slightly more common among those with a household income of less than $10,000 compared to those with a higher household income in both men and women . In the US ARIC study, a household income of less than $16,000 vs. $75,000 and more was associated with a slightly increased prevalence for migraine with and without aura, but not with other headache types . In contrast, in a survey from Philadelphia County, which shows demographic similarities with the general US population, no association between income and migraine prevalence was found; neither for the crude nor for the age-adjusted prevalence . In a recent study again an increased headache frequency in the group with the lowest annual income (<$22,500) was reported . Pryse-Phillips et al., in a Canadian survey on migraine and tension-type headache, could otherwise not find any correlation between these headache types and household income . In contrast to most of the studies from the USA, in our study the headache period prevalence was constantly slightly higher at a monthly income of 2,500€ or more than with lower monthly income. Two other German surveys also show a higher migraine period prevalence in the medium and higher socio-economic groups [5, 6]. The reasons for these different results remain unclear, but it should be taken into consideration that the social systems in Europe and USA are quite different.
The prevalence show a distinct dependence on sex and age, which results in a stable, almost identical bell-shaped age distribution over all investigated years. Women always showed higher one-year prevalence rates than men in all age groups. These dependencies have been described repeatedly for migraine [1, 2, 37], for headaches in general, however, less data is available. Since tension-type headaches are more frequent than migraine in all the samples, the reported age dependence indicates that it is also applicable to tension-type headache and not only to migraine. This corresponds with the results from regional studies (Baltimore County in USA, Nord-Trondelag County in Norway) [38, 39]. On the other hand, a nationwide representative study in France showed period prevalence of non-migraine headaches to be age- or sex-dependent only for women younger than 25 years . Boardman et al. discuss possible reasons for this age-dependent decrease in period prevalence of headaches, for example a possible recall bias due to the increase of other more relevant diseases among the elderly . Also, Russell et al. ask whether the age-dependent decrease, which is statistically significant in their survey only for men, but not for women, is a real phenomenon or caused by recall bias . However, a 15-year cohort study with employees of a French gas and power supply company showed a clear decrease of the prevalence of headaches at the point of retirement of the employees, with the decrease being particularly distinct in the group of employees with high work-related stress .
The interpretation of the results should take the strengths and limitations of this study into account. The choice of interview method depends essentially on the contents of the information to be gathered . Much more complex information can be collected in face-to-face interviews than by mail surveys . Telephone interviews tend to be much shorter, e.g. in a Spanish telephone survey on pain, the interview lasted 10.5 minutes on average , while it took about 90 minutes in the French GRIM2000 face-to-face migraine survey  and 50 minutes in our study. Face-to-face interviews achieve the highest response rates and are terminated early by the interviewees less often than telephone surveys or mail surveys [43, 44]. In our study, the response rate of 67.4% to 73.1% was higher than that of the DMKG survey with 66.9%  and about as high as in a regional Croatian face-to-face survey . It was also higher than that of surveys with other data collection methods, such as that of 53.3% in the GNTHIS 2004 (telephone interviews)  or that of 62% in a US telephone survey , or 37.1% in a Spanish telephone survey . The most important aspect of our study is the large number of interviewees with more than 12,000 interviews per year over a total of 15 years, i.e. a database of more than 146,000 analyzed face-to-face interviews. Furthermore, the multi-topic survey contributed at least partly to the required “blinding” of the topic and helped to minimize a recall bias . The method used ensured that there was a standardized interview situation over the whole period [43, 44]. One of the limitations under discussion in some surveys is the lack of ICHD-II classification of the headaches and that the interviews were not done by headache experts. Nevertheless, Rasmussen points out that a physician might subconsciously influence patient responses into patterns corresponding to his ideas of IHS headache classification . Even with adequate training and the use of structured interviews, misclassification cannot be excluded completely and an underreporting of migraine might still be possible. As this survey only asked about the presence of any kind of headache, independent of its type, intensity, frequency, and duration, the use of trained interviewers is not a relevant limitation in our survey. Moreover, this kind of question and the restriction to one year could minimize recall bias, a serious problem of retrospective surveys. The study was not designed to evaluate risk factors for headache and we therefore did not apply multivariate analyses.