## 4 Dataset
### Ghost Hotels  
The main focus when studying STR impact on the housing markets is around entire home or apartments listings which, as it has already been discussed in Section 4.1 (Table 2), account for the overwhelming majority of the available properties in the City of Athens. The reason is that the aforementioned listings can no longer be available to house long-term tenants, thereby intensifying the housing crisis that has been witnessed in Athens in recent years. Private or shared rooms, on the other hand, are generally not regarded as a contributing factor to the said phenomenon, as they are viewed as properties that don't affect the housing availability for regular tenants.  
Nevertheless, a detailed inspection on private or shared rooms reveals that above assumption is not always valid. Table 5 groups these categories of rooms on a per host basis, for the first and last data dumps of the dataset (Section 4). For example, on 17 th July 2015, 387 private or shared rooms were available on Airbnb for the City of Athens, 228 of which were listed by hosts having exactly one listing on the platform and the rest by hosts having more listings. This last grouping of listings are generally being referred to as \"ghost hotels\" because a single host pretends to possess multiple small properties while, in fact, s/he owns a larger one, split into individual rooms, much like an ordinary hotel.  
In reality, these cases constitute an unregulated form of hotel operation and have been repeatedly labeled as unfair competition by both registered hotel owners and tourism-related authorities [47]. Monitoring the existence and expansion of ghost hotels is of great importance as it illustrates the dynamics of STRs and the development of tourism within the city. Indeed, Table 5 also portrays a more than a three-fold increase in relevant entries between the first and last data dumps, while ghost hotel listings have witnessed a 6-fold increase from 97 to 789 or from 40% to 70%.  
Figure 3 illustrates ghost-hotel expansion in the City of Athens between 2015 & 2019 (first & last data dumps). It is very similar to Figure 1, in the sense that most ghost exist in the central neighborhoods and not in the periphery. However, ghost hotel prevalence does not follow the same patterns as entire homes and apartments, since it does not seem to move to southern, northern & south-eastern areas. An exception to this rule is the rather sharp density increase around the areas of the Central Railway Station of Athens (Stathmos Larissis) and in the Kypseli neighborhood.  
\begin{table}
\begin{tabular}{l r r r r} \hline \hline \multirow{2}{*}{**Number of Listings per Host**} & \multicolumn{2}{c}{**17 th July 2015**} & \multicolumn{2}{c}{**19 th November 2019**} \\ \cline{2-5}  & **Listings** & **Percentage** & **Listings** & **Percentage** \\ \hline One listing & 228 & 58.91\% & 430 & 30.96\% \\ More listings & 159 & 41.09\% & 959 & 69.04\% \\ Total & 387 & 100.00\% & 1389 & 100.00\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Number of shared/private/hotel rooms listings per host.  
Figure 3: Change of ghost hotel density of Airbnb listings in Athens neighborhoods between 2015 and 2019 (color online).