# Topology of the World Tourism Web
## 2 Materials and Methods
### Definitions and Data
The same problem of the data collection has been addressed by the World Bank, as it used the same sources for representation of [43], meaning that the countries only loosely adhere to recommendations for tourism statistics [44] in their reports. From the perspective of the time periods in the available time series, the data for the latest year available for most countries (187 countries) are for 2018, so we focused on the data from that year. There were 14 countries for which we could not find data for any of the stated type, neither in the UNWTO database, nor on their national statistics bureaus (or tourist bureaus, if any) websites. In addition, there were 23 countries with outdated data, meaning that the reports exist but do not include the target year. The systematization of the countries according to the source of the data used in the analysis is listed in Appendix A, Table A1, but the data were also published in a yearbook [45]. We used the following available data: arrivals of tourists at national borders, by country of residence as the preferred source; if unavailable, then arrivals of tourists at national borders, by nationality; if unavailable, then arrivals of non-resident tourists in all types of accommodation establishments, by country of residence/nationality; if unavailable, then arrivals of non-resident tourists in hotels and similar establishments, by country of residence/nationality; if unavailable, then arrivals of non-resident visitors at national borders, by country of residence/nationality; if unavailable, then data from the statistics bureau (or the tourist bureau) of the country. For countries for which only data for earlier years were available, we approximated the values for 2018 (for the countries where aggregated data for 2018 were unavailable, we used the time series from the International Tourism, Number of Arrivals document and extrapolated the expected value for 2018 from the trend; to approximate the number of arrivals for each country, we used the shares of the last available year and applied them to the extrapolated value). For countries that reported unusual data, we additionally checked the data from national statistics bureaus and used those data (for example, for Poland, the data reported as tourist arrivals at national borders were actually the number of overnight stays). In addition, some countries' reports appeared unusual, but the control revealed that they keep the statistics in thousands (for example, Ireland and the Netherlands). Another limitation arises from the data that referred to the Other category, where some countries report a substantial number of arrivals, while probably focusing only on the arrivals from the countries they deem relevant. From the SNA perspective, this represents a significant loss of information, which could result in structural holes, even though they may not be present otherwise. However, besides the recommendation for a detailed and systematized approach to data collection in tourism, from the current research perspective, we cannot compensate for that occurrence. As previous studies on the topic [6; 7] do not mention difficulties in data collection and the variety of approaches (besides the unavailability of data for some countries), we can only assume that the procedure of data collection in tourism has worsened and recommend the formation of a detailed and unified concept of data collection. As the exclusion of the nodes in the network leads to an exponential loss in information [19], we strive to include all the available data. For example, exclusion of one node from this network in some cases could result in exclusion of up to 400 links. However, while the methodological inconsistencies suggest the need for a unified approach to data collection, they also represent a limitation for this research. An additional limitation is the snapshot approach, considering only one year. Nevertheless, such approach still enables approximation of the world tourism web, and it has been applied before [6,7].