According to recent changes of medical service from treatment focus to prevention focus, the dominant market of providing treatment services is being replaced by the market of consumers visiting hospitals to receive prevention services [1]. Also clients now select hospitals more freely by collecting information from various sources and therefore, hospitals have a higher threat to their business environment due to leaving patients [2].

So far, general companies and hospitals have put in much effort to attract new customers for management improvement. Awareness of existing customers was relatively small and as a result the hospitals are experiencing many leaving existing customers [1,3,4]. According to some studies, the expenses for attracting new customers have been about five times as high as the expenses for maintaining existing customers [5], and a 5% reduction of customer defection rate of regular customers has effected the increasing company net profit by 25-85% [6]. In this view, it appears that maintaining existing customers is favorable than creating new customers in terms of a strategic hospital management. As a result, rather than using a customer acquisition strategy towards unspecified individuals, now hospitals select loyal customers by analyzing accumulated information with those who have maintained a relationship for a long period of time and are introducing customer relationship management (CRM) marketing regarding them [7,8]. In other words, for management deterioration due to customers leaving and opportunity costs, CRM is becoming an important issue to the hospitals management.

CRM is a process of marketing by segmenting customers to better understand them and for the purpose of improving long-term relationships with valuable customers [8]. That is, a most distinct feature of CRM is not a traditional method of collecting the most number of customers but, CRM is a customer centered marketing which provides a service that meets individuals based on their characteristics and consuming patterns [6]. This technique of marketing arose from the segmentation, targeting, positioning (STP) strategy which can be seen as the core of marketing and STP separates a market of large-scale customers (segmentation) and selects the target market (targeting) and then positioning a service or product into their minds for recognition (positioning). In other words, segmentation, targeting, and positioning together comprise a three stage process; first, determining which kinds of customers exist, second, selecting which ones corporations are best off trying to serve and, finally, implementing their segmentation by optimizing their products/services for that segment [9]. Therefore, in order to perform CRM, it is extremely important to select targets that the hospitals can provide intensive services, discover high fidelity customers with an accurate understanding their characteristics, and further, predicting fidelity customers is necessary.

This study was performed to suggest a practical method of data-mining in CRM of hospitals. A detailed research aims are discovering loyal customers from a large scale database of discharged patients by combining data-mining with STP strategy and recency, frequency, monetary (RFM) model that are being used as marketing strategies among general companies. RFM model is used for customer value analysis and applied for market segmentation. It is a behavior-based model to analyze the customers' purchasing patterns by using customers' information in large scale database. RFM model is composed of three measures, namely recency, frequency, and monetary [10-12].

This study used a database of discharged patients from a university hospital in Seoul between January 1st and December 31st 2009. Among a total of 16,346 discharged patients, we excluded unsuitable patients for this study purpose (younger than 19, foreigners, and patients who were participating clinical trials), and 14,072 patients were selected as final subjects.

In this study, STP strategy and RFM model are combined with data-mining in order to perform CRM of hospitals. In order to discover fidelity customers, segmentation and targeting was performed by applying a core marketing strategy, i.e., STP strategy [9] and while using this, variables of RFM model were used, which has been used for the criteria of segmentation in marketing. It is also being used as an extremely important method when commencing marketing activity or assessing customers' values [10,11]. And more recently, this is being used as a classification method of fidelity customers to perform CRM in hospitals [12]. In this study, the variables representing consuming frequency was the number of admission and visiting out-patient department (OPD) prior to one year of index admission and the variables representing monetary were expenses for being discharged from the hospital and the expenses per visit for out-patient care.

Independent variables consisted of the factors associated with loyal customers revealed in previous study [1,12]. And it can be classified into three characteristics: demographic characteristics, medical service use characteristics and disease group characteristics. First, the variables of demographic characteristics are gender, age, place of residence, and insurance, and the variables of medical service use characteristics are a department (internal/surgical affiliation), kind of patient room, route of admission, surgery certainty, readmission certainty, length of stay (LOS), and a number of accompanying treatments. Also, International Classification of Diseases (ICD)-10 code of main diagnostic criteria at the time of discharge was used for disease group characteristics.

The analyzing method for market segmentation and target market selection, k-means algorithm cluster analysis was performed by applying RFM variables and comparing the groups' differences through a t-test. Furthermore, to compare target market of fidelity customers and general customers' demographic characteristics, medical service use characteristics, disease group characteristics, t-test and chi-square test were performed. Lastly, the medical use pattern modeling was performed by applying a decision tree. During this process, the full analysis of the data was segmented to 70% of training data and 30% of validation data. This study relied on the training data to create a model; it applied the created model to the validation data [13-15].

To assess the decision tree, it has been compared with a logistic regression by using root asymptotic standard error (ASE), misclassification rate and receiver operating characteristic (ROC) curve which are the most fundamentally used for comparing the predictive power of models.

For data analysis, t-test and chi-square tests were performed using the statistical package SAS ver. 9.1 (SAS Institute Inc., Cary, NC, USA) and the cluster analysis and decision tree used the Enterprise Miner ver. 4.0 (SAS Institute Inc.).

For selected loyal customers through market segmentation, a medical use pattern model of loyal customers was established by using a decision tree which is one of the data-mining methods. A decision tree is a set rule that predicts target variables and creating the classification trees by repeatedly dividing the data. During this process, a tree branch is made and every branch decides the classification criteria of the dividing data. Therefore, it explores the set of data and determines the variable that is predicted as the most significant of the predictor variable. Structure of the decision tree provides good visualization, and helps understand and explain the process with ease, but its weakness is that when the sample size is small there are a large number of end branches formed [13,14,16]. In this study, by applying the data 70% of training and 30% of validation, a model overfitting that can be occur during estimating the model was solved [13,15]. And to evaluate decision tree model, it was compared with the logistic regression model, which is fundamentally used when the dependent variable is binomial.

The decision tree showed that the most important factor of classifying loyal customers is LOS and when they were relatively higher than the criteria based on 13.5 days, there was a higher probability of being loyal customers. A study on the model of predicting expenses of patients who were admitted in the hospital showed that the most important factor in predicting expenses was the LOS (by standards of 14.5 days). This is because hospitalized treatments expenses were included mostly with patients' rooms fees and this increases with the prolongation of hospitalized days [17,18]. Even in this report, longer LOS increased profitability of patients due to their hospitalized treatments fees and implying that they have a higher probability of being selected as loyal customers.

In case of the LOS was 13.5 days or longer, a factor that classifies the loyal customer was certainty of surgery, when one had a surgical operation, the probability of being a loyal customer was 41.1%. In previous study, the number of operations was shown as a factor of affecting increase in treatments fees and this is because the increase in profitability in respect to increase in treatments fees of patients due to surgical operation [17]. In a case of 13.5 days or longer LOS with a surgery, the factor of classifying into loyal customers was kind of the patient's room and when patient stayed in the private room, the probability of being loyal customers was high (46.2%). Room type is an important factor of affecting expenses for inpatient like LOS because treatment fees increase with using higher class rooms [18]. In the case of patients whose LOS were 13.5 days or longer and has not been operated on, then the loyal customer classifying factor was department from which they were discharged. When a patient was treated in an internal medicine department, the probability of being a loyal customer was high (70.0%). Patients from the internal medicine department is generally higher due to the relatively high proportion of patients with chronic disease and complications, hence it was reported that they have higher frequency of medical use [19].

In the case where LOS was shorter than 13.5 days, the factor of classifying loyal customers was certainty of selectable treatments and the customers who had selected treatments had higher chance of being loyal customers (28.0%). In previous study, the loyalty of customers was shown as being affected by selectable treatments [1]. In selectable treatments, patients choose doctors who they want to receive treatments from and are based on the relationship of the doctor and patient. Moreover, the relationship with doctors positively affects the hospital's impression on customers. So customers who choose treatments than those who choose not to receive them have a higher credibility regarding the hospital and doctors also increasing their loyalty.

The limitation of this research is first, subjects are limited to only one hospital so the results cannot be generalized. Secondly, a variable that represents a degree of severity of patients was not included. In order to compromise this, surgery certainty and the number of accompanying treatments were used as independent variables yet it is hard to determine the severity of the disease directly. Thirdly, the proportion of unpaid treatment fees was a variable that represented the loyalty of customers in the previous study [1] but this study was unable to confirm unpaid treatment fees.

Despite the above limitations, the significance of this study is that is suggested an application method of data-mining for a hospital's CRM. To distinguish loyal customer and by modeling their medical use pattern, classification and prediction which are data-mining's important functions were applied to a hospital's marketing. A successful marketer must identify the needs of customers in different segments [12]. This study used RFM model and STP strategy, which are generally used in marketing, to classify patients and target loyal customers. This paper suggests practical use of combining STP strategy and the RFM model with data-mining in CRM. And medical use patterns of loyal customers were established in modeling by applying a decision tree so, it assisted in not just finding factors associated with loyal customers but also to find increase loyal customers' predictable probability in steps according to each factors.

Through the hospital CRM that was introduced in this study along with the uses of the data-mining method, we look forward to helping hospitals that are faced with recently increasing competitions improve their management activities.