preliminaries: Addressed Katerina's comments

text/preliminaries/data.tex

@@ -13,6 +13,19 @@ We firstly classify data based on their content \kat{'based on their content' re
   \item \emph{Statistical data}---the outcome of statistical processes on microdata.
 \end{itemize}
 
+To accompany and facilitate the descriptions in this chapter, we provide the following running example.
+
+\begin{example}
+  \label{ex:snapshot}
+  Users interact with an LBS by making queries in order to retrieve some useful location-based information or just reporting user-state at various locations.
+  This user--LBS interaction generates user-related data, organized in a  schema with the following attributes: \emph{Name} (the unique identifier of the table), \emph{Age}, \emph{Location}, and \emph{Status} (Table~\ref{tab:snapshot-micro}).
+  The `Status' attribute includes information that characterizes the user's state or the query itself, and its value varies according to the service functionality.
+  Subsequently, the generated data are aggregated (by issuing count queries over them) in order to derive useful information about the popularity of the venues during the day (Table~\ref{tab:snapshot-statistical}).
+
+  \includetable{snapshot}
+
+\end{example}
+
 \kat{I miss the definition of data. You speak of data items, data values, what is the difference to data?}
 An example of microdata is displayed in Table~\ref{tab:snapshot-micro}, while an example of statistical data in Table~\ref{tab:snapshot-statistical}.
 Data, in either of these two forms, may have a special property called~\emph{continuity}, i.e.,~their values change and can be observed through time. \kat{The way that you define it here reminds temporal data. What is the difference?}