Patent Publication Number: US-2017347884-A1

Title: Correlation determination method and correlation determination apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2015/082915 filed on Nov. 24, 2015 and designated the U.S., the entire contents of which are herein incorporated by reference, and which is based upon and claims the benefit of priority to Japanese Patent Application No. 2015-035389 filed on Feb. 25, 2015, the entire contents of which are herein incorporated by reference. 
    
    
     FIELD 
     A certain aspect of the embodiments discussed herein relates to a correlation determination method, and a correlation determination apparatus. 
     BACKGROUND 
     In medical practice, treatments are generally conducted based on observational information in hospitals. 
     With the advent of wearable sensors and the like, computers are now enabled to collect events (hereinafter referred to as “events”) indicated by sensor data (accelerations, speeds, heart rates, blood pressures, etc.) relating to activities during daily life for each individual. Further, individuals or physicians, etc. may input, into a computer, events (drug administration, dietary intake, etc.) relating to activities during daily life. 
     As described above, observation of collected or input events with respect to activities of patients during daily life may enable physicians to clarify conditions or circumstances that are likely to cause functional abnormality in the patients, leading to potentially effective medical treatments. For example, a physician may be able to clarify that abnormality in a patient&#39;s blood pressure takes place after the patient takes a certain drug, and the physician may discontinue the drug as the effective treatment for this condition. 
     In general, it is assumed that an event to be focused on is specified for each function abnormality (hereinafter referred to as “focused event”) based on typical medical knowledge. For example, in a case of a patient with abnormality in the heart, one needs focus on the abnormality of the electrocardiogram waveform, or in a case of finding risks of falling, one needs to focus such as on a decrease in walking speed or on a staggering gait occurrence. 
     Focusing on such an event may enable extraction of some background of a cause of the focused event, that is, another event correlated with the focused event (hereinafter referred to as a “related event”), which may provide effective knowledge for remediating the functional abnormality. 
     With respect to events related to computer systems, attempts have been made to conduct a study of extracting, as a related event, an event for which the occurrence frequency immediately before the focused event is equal to or higher than a threshold. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-open Patent Publication No. 2008-41041 
         Patent Document 2: Japanese Laid-open Patent Publication No. 2004-157614 
         Patent Document 3: Japanese Laid-open Patent Publication No. 2007-48200 
         Patent Document 4: Japanese Laid-open Patent Publication No. 2013-131170 
         Patent Document 5: Japanese Patent Application No. 2001-540710 
         Patent Document 6: Japanese Laid-open Patent Publication No. 2011-209908 
       
    
     However, with respect to activities during the daily life of a person, in particular, a timing of occurrence of a related event is not necessarily immediately before the focused event. For example, blood pressure may decrease between 5 and 60 minutes after meals. Further, it may take between 5 and 15 minutes to return to a normal heart rate after exercise. In addition, heart rate may rise in 30 minutes to 90 minutes after a meal, due to energy consumption by digestion. 
     As described above, there is a high possibility of failing to extract related events having a long occurrence interval with respect to the focused event only by observing a period immediately before the focused event. 
     In view of this, in order to enable extraction of related events having long occurrence intervals, consideration may be made to extract, as a related event, an event whose occurrence frequency in a long term range from the occurrence of the focused event is equal to or higher than a threshold. However, in this case, events related to actions normally performed as daily activities may be extracted undesirably. For example, with respect to a person having a lifestyle habit of going to bed within 5 hours after having dinner, there is a possibility of extracting dinner intake as a related event in a case of going to bed being determined as a focused event. 
     In addition, in a case where a related event is extracted depending only on the occurrence frequency of the related event, there is a high possibility of extracting constantly occurring events, such as “breathing” and “blinking”, as related events of a certain event. 
     According to an aspect of the embodiments, a correlation determination program for causing a computer to execute a process is provided. The process includes calculating a time interval between an occurrence time of one of first type events and an occurrence time of each of second type events based on information including the occurrence times of the first type events and the second type events, on a first type event basis, each of the second type events occurring within a predetermined time from a corresponding one of the occurrence times of the first type events; comparing, for each of a plurality of ranges of values relating to the time interval, the plurality of ranges differing from one another in at least one of a minimum value and a maximum value, a first rate of the number of time intervals within a corresponding one of ranges among a set of time intervals with respect to a size of the range and a second rate of the number of time intervals outside the corresponding range among the set of time intervals with respect to a size of a range obtained by excluding the range from the predetermined time; and determining presence or absence of a correlation between the first type event and the second type event. 
     SUMMARY 
     According to an aspect of the embodiments, a non-transitory computer-readable recording medium having stored therein a correlation determination program for causing a computer to execute a process is provided. The process includes calculating a time interval between an occurrence time of one of first type events and an occurrence time of each of second type events based on information including the occurrence times of the first type events and the second type events, on a first type event basis, each of the second type events occurring within a predetermined time from a corresponding one of the occurrence times of the first type events; comparing, for each of a plurality of ranges of values relating to the time interval, the plurality of ranges differing from one another in at least one of a minimum value and a maximum value, a first rate of the number of time intervals within a corresponding one of ranges among a set of time intervals with respect to a size of the range and a second rate of the number of time intervals outside the corresponding range among the set of time intervals with respect to a size of a range obtained by excluding the range from the predetermined time; and determining presence or absence of a correlation between the first type event and the second type event. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are diagrams illustrating a method of reading drawings used in present embodiments; 
         FIG. 2  is a diagram illustrating a first idea acting as a basis in an embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a second idea acting as a basis in an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating a third idea acting as a basis in an embodiment of the present invention; 
         FIGS. 5A and 5B  are first diagrams illustrating an outline of a related event determination method in an embodiment of the present invention; 
         FIG. 6  is a second diagram illustrating an outline of the related event determination method according to the embodiment of the present invention; 
         FIG. 7  is a diagram illustrating a hardware configuration example of a correlation determination apparatus according to an embodiment of the present invention; 
         FIG. 8  is a diagram illustrating a functional configuration example of the correlation determination apparatus according to the embodiment of the present invention; 
         FIG. 9  is a flowchart illustrating an example of a process executed by a correlation determination apparatus; 
         FIGS. 10A and 10B  are diagrams illustrating examples of respective time series data of a focused event and a candidate event; 
         FIG. 11  is a diagram illustrating a configuration example of a parameter storage; 
         FIG. 12  is a diagram illustrating an example of a calculation result of a time interval between a focused event and a candidate event; 
         FIG. 13  is a diagram illustrating generation of a determination window; 
         FIG. 14  is a diagram illustrating an example of data specified by a generation process of a determination window; 
         FIG. 15  is a flowchart illustrating an example of a process of determining whether there is a correlation between a focused event and a candidate event with respect to one determination window; 
         FIG. 16  is a diagram illustrating a calculation example of Z1 in  and Z1 out  and a ratio R1 for each case; 
         FIG. 17  is a diagram illustrating an example of a method of obtaining a threshold T_TH according to a start point of a determination window; 
         FIG. 18  is a diagram illustrating an example of a method of calculating a ratio R2; and 
         FIG. 19  is a diagram illustrating a configuration example of a related event storage. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. First, the way of reading the drawings used in the present embodiments will be described.  FIGS. 1A and 1B  are diagrams illustrating a method of reading drawings used in the present embodiment. 
     In the present embodiments, it is assumed that data relating to activities in daily life of a person are detected by a sensor attached to the person or carried by the person, or input by the person, and such data are collected by a computer. In such a condition, a method of extracting an event having a correlation or relationship with a certain event (hereinafter unified as the term “correlation”) will be described. Note that a “certain event” hereinafter is referred to as a “focused event”, and an event correlated with the focused event is referred to as a “related event”. 
     To extract a related event, an event subject to being determined as a focused event or a related event (hereinafter referred to as “candidate event”) is initially specified. 
       FIG. 1A  illustrates an example in which an occurrence point of a candidate event and an occurrence point of a focused event, both of which are specified based on collected data, are arranged on a timeline. That is, a horizontal direction of  FIG. 1A  indicates a time axis, and time progresses from left to right. In each row, a point at which a filled rectangle is arranged indicates the occurrence point of the candidate event or the focused event. In the present embodiment, each occurrence of a focused event is referred to as a “case n”. Further, “n” indicates the occurrence order of the focused events in a period where the focused events are subject to observation. In  FIG. 1A , cases 1 to 3 are illustrated, and cases subsequent as of a case 4 are omitted. 
     In  FIG. 1B , for each case n, a candidate event that has occurred within a predetermined period from the occurrence point of the case n is plotted on the basis of the occurrence point of the case n. That is, in  FIG. 1B , each case is arranged in a vertical direction. In a row for each case n, the occurrence point of the corresponding candidate event, which has occurred within a predetermined period from the occurrence point of a focused event relating to the case n, is plotted. For example, as apparent from a range indicated by a broken-line rectangle in  FIG. 1A , the candidate event occurs twice in a predetermined period before the occurrence of the case 2. Accordingly, in  FIG. 1B , the occurrence points of these two candidate events are plotted, respectively, in the row corresponding to case 2. 
     Further, in  FIG. 1B , a solid-line rectangle indicated by a symbol W is a window (hereinafter referred to as “determination window”) indicating a duration in which a candidate event is subject to observation. In the present embodiment, while shifting the determination window W in a horizontal direction, the occurrence frequency (hereinafter referred to as unit “frequency”) of candidate events in the determination window W is measured. The formula for calculating the frequency of certain events is, for example, as follows. 
     Frequency=(the number of cases in which an event occurred one or more times within the determination window W)÷the number of all cases. For example, the frequency of candidate events in  FIG. 1B  is 1÷8=0.125. Note that, in the determination window W, even if the candidate event were to occur twice with respect to Case 5, the frequency of the candidate event would still be 0.125. 
     Next, the basic idea in the present embodiment will be described.  FIG. 2  is a diagram illustrating a first idea acting as a basis in the embodiment of the present invention. 
     The first idea determines that the correlation between the candidate event and the focused event is high in a case of the frequency in the determination window W being much larger than the frequency outside the determination window W, even if the frequency in the determination window W is small. That is, in  FIG. 2 , the frequency in the determination window W is the same in a left side condition and a right side condition, but the ratio of the frequency in the determination window W to the frequency outside the determination window W is higher in the right side condition. Accordingly, in  FIG. 2 , a correlation between the candidate event and the focused event is considered to be higher in the right side condition than the left side condition. 
     For example, it is assumed that the focused event is the start of sleeping and the candidate event is the end of a meal. In the left side condition, the frequency of starting to sleep between 30 to 45 minutes from after meals (a time period of the determination window W) is 4÷5=0.8, and a similar frequency is observed in other time periods. However, in the right side condition, although the frequency of starting to sleep between 30 and 45 minutes (a time period of the determination window W) from after meals is also 4÷5=0.8, a similar frequency is not observed in other time periods. In such a case, it is considered that the correlation between the end of a meal and the start of sleep is higher in the right side condition. 
     However, according to the first idea, as the duration of the determination window W is wider, there may be a high possibility that the frequency within the determination window W will increase. Hence, in order to supplement the first idea, a second idea is introduced in this embodiment. 
       FIG. 3  is a diagram illustrating the second idea acting as a basis in the embodiment of the present invention. The second idea indicates a case where the difference is small between the frequency of the determination window W having a relatively narrow duration and the frequency of the determination window W having a relatively wide duration, and determines that the former has higher correlation even if the frequency of the former is smaller. 
     For example, in  FIG. 3 , the frequency in the determination window W in the left side condition is higher than the frequency in the determination window W in the right side condition; however, a width of the determination window W in the right side condition is much narrower than a width of the determination window W in the left side condition. Hence, in this case, the correlation is considered to be higher in the determination window W in the right side condition. The determination window W in the left side condition illustrates that the frequency of starting to sleep between 0 to 300 minutes after meals is high. The determination window W in the left side condition illustrates that the frequency of starting to sleep between 30 to 45 minutes after meals is high. In such a case, it is considered that the correlation between the end of a meal and the start of sleep is higher in the right side condition. 
     However, as an interval increases between a time at which a related event occurs and a time at which the focused event occurs, variability in the occurrence time of the related event with respect to the focused event in each case will increase. For example, when a case where side effect X develops within 5 minutes after taking medicine A is compared with a case where side effect Y develops 3 hours after taking medicine B, the latter case has high possibility of having variability in timing of developing the side effect. This is because physiological characteristics of a person, such as a physical condition, may vary with the day when the medicine is taken. Further, the longer the interval between the related event and the focused event, the higher the possibility of adding another action that increases the interval between the related event and the focused event. For example, in comparing people who go to bed within 30 minutes after taking medicine, and people who go to bed after 3 hours after taking medicine, the latter people may have high possibility of varying time going to bed due to activities performed during 3 hours. 
     With the first and second ideas alone, for example, even if the correlation between the above “taking medicine A” and “development of side effect X” and the correlation between taking medication B″ and “development of side effect Y” is the same degree as such, there may be a possibility that the former is determined to have a higher correlation. Hence, in order to supplement the second idea, a third idea is introduced in this embodiment. 
       FIG. 4  is a diagram illustrating the third idea acting as a basis in the embodiment of the present invention. The third idea estimates a possibility of having a correlation between the candidate event and the focused event when the duration of the determination window W is large and the interval between the candidate event and the focused event is substantially long. 
     In a case of the left side condition of  FIG. 4 , the frequency of candidate events in the determination window W is 1/5=0.2. However, in  FIG. 4 , since the candidate event group is about 90 minutes separate from the focused event group, as illustrated on the right side of  FIG. 4 , the candidate event group is allowed to vary and the duration of the determination window W is increased; as a result, the frequency of candidate events in the determination window W is 4÷5=0.8. 
     In the present embodiment, it is determined whether the candidate event is a related event based on the above-described first to third ideas. Next, the points of determining the related event in the present embodiment will be described. 
       FIGS. 5A and 5B  are first diagrams illustrating an outline of a related event determination method in an embodiment of the present invention. 
     In this embodiment, two or more types of determination windows W mutually differing in at least one of the duration and the position are used. For each determination window W, the occurrence rate of the candidate events within the determination window W and the occurrence rate of the candidate events outside the determination window W are calculated. In any of the determination windows W, when the ratio of the former occurrence rate (candidate event within the determination window W) to the latter occurrence rate (candidate event outside the determination window W) is equal to or greater than the threshold R_TH, the candidate event is determined to be a related event. Note that the occurrence rate of candidate events within the determination window W is calculated by dividing the number of occurrences of candidate events in the determination window W by the duration of the determination window W. Further, the occurrence rate of the candidate events outside the determination window W is calculated by dividing the number of occurrences of the candidate event outside the determination window W by a period obtained by subtracting the duration of the determination window W from the overall period (a period in a horizontal direction in  FIGS. 5A and 5B ). Accordingly, with respect to  FIGS. 5A and 5B , there is a higher possibility of determining in  FIG. 5B  that the candidate event is a related event than in  FIG. 5A . Note that the process described in  FIGS. 5A and 5B  is referred to as “point  1 ”. 
     However, depending on the occurrence condition of the event, there are cases in which point  1  may not be valid. To supplement such a case in the present embodiment, a process described in  FIG. 6  is executed. 
       FIG. 6  is a second diagram illustrating an outline of the related event determination method according to the embodiment of the present invention. In  FIG. 6 , a candidate event is assumed to have occurred 100 times in a case 1. In contrast, the candidate event is assumed to have occurred only once in each of cases 2 to 4. In such a case, there is a high possibility of calculating the ratio by point  1  to be less than the threshold R_TH. Accordingly, there is a high possibility of determining the candidate event being not a related event. However, in observing cases 2 to 4, the possibility of the candidate event being a related event is considered to be very high. 
     Hence, in the present embodiment, before the execution of point  1 , the occurrence rate of the candidate events within the determination window W and the occurrence rate of the candidate events outside the determination window W are calculated for each case. As a result, when a ratio of the cases, for which the ratio of the former occurrence rate to the latter occurrence rate is equal to or greater than a threshold α, is equal to or greater than a threshold β, point  1  is not executed and the candidate event is determined to be a related event. 
     The process described in  FIG. 6  is referred to as “point  2 ”. Point  1  and Point  2  are processes based on the first idea. 
     In the present embodiment, two or more types of determination windows W mutually differing in at least one of a duration and a position are used; however point  1  is not valid with respect to the determination windows W of all the durations. That is, as described with reference to  FIG. 3 , even when the ratio by point  1  is the same with respect to the determination window W having a short duration and the determination window W having a long duration, the ratio with respect to the determination window W having a short duration needs to be evaluated as having a high correlation. 
     Hence, in the present embodiment, in point  2 , when the candidate event is not determined to be a related event and the duration of the determination window W is equal to or less than the threshold T_TH, a process called point  3  is executed to enable the process of point  1  to be valid. Note that point  3  is a process corresponding to the third idea. 
     Furthermore, as described with reference to  FIG. 4 , the longer the interval between the focused event and the related event, the greater the allowable range should be with respect to the deviation between the candidate events. Hence, in the present embodiment, the threshold T_TH used in point  2  is changed according to the position of the determination window W. More specifically, as the position of the determination window W becomes farther from the focused event, the threshold HT 2  becomes larger. Hereinafter, the process related to the change of the threshold T_TH is referred to as point  4 . 
     The following illustrates a correlation determination apparatus  10  configured to determine related events based on the points  1  to  4 . 
       FIG. 7  is a diagram illustrating an example of a hardware configuration of a correlation determination apparatus according to an embodiment of the present invention. The correlation determination apparatus  10  in  FIG. 7  includes a drive device  100 , an auxiliary storage device  102 , a memory device  103 , a CPU  104 , an interface device  105 , and the like. 
     A program for implementing a process in the correlation determination apparatus  10  is provided from a recording medium  101 . When the recording medium  101  storing the program is set in the drive device  100 , the program is installed from the recording medium  101  to the auxiliary storage device  102  via the drive device  100 . Note that it is not always necessary to install the program from the recording medium  101 ; however, the program may be downloaded from another computer via the network. The auxiliary storage device  102  stores the installed programs and stores necessary files, data, and the like. 
     The memory device  103  reads a program from the auxiliary storage device  102  and stores the read program when receiving an instruction to activate the program. The CPU  104  implements a function of the correlation determination apparatus  10  according to the program stored in the memory device  103 . The interface device  105  is used as an interface for connecting to the network. 
     Note that an example of the recording medium  101  is a removable recording medium such as a CD-ROM, a DVD disk, or a USB memory. Further, examples of the auxiliary storage device  102  include an HDD (Hard Disk Drive), a flash memory, and the like. Each of the recording medium  101  and the auxiliary storage device  102  corresponds to a computer-readable recording medium. 
       FIG. 8  is a diagram illustrating a functional configuration example of the correlation determination apparatus according to the embodiment of the present invention. In  FIG. 8 , the correlation determination apparatus  10  includes an event selector  11 , an event data acquisition unit  12 , a parameter acquisition unit  13 , a time interval calculator  14 , a determination window generator  15 , a correlation determination unit  16 , a result output unit  17 , and the like. These units are implemented by a process to be executed by the CPU  104 , which is caused by one or more programs installed in the correlation determining apparatus  10 . The correlation determination apparatus  10  also uses an event data storage  121 , a parameter storage  122 , a related event storage  123 , and the like. Each of these storages may be implemented by using, for example, the auxiliary storage device  102 , or a storage device or the like connectable to the correlation determination apparatus  10  via a network. 
     The event selector  11  selects a type of an event to be the focused event and a type of an event to be the candidate event. 
     The event data acquisition unit  12  acquires event data on the focused event and event data on the candidate event from the event data storage  121 . The event data storage  121  stores event data on each of the events in time series. Each of event data includes, for example, the type of the event, the occurrence date and time of the event, and the like. 
     The parameter acquisition unit  13  acquires from the parameter storage  122  parameters related to the determination window W used for the process of determining the presence or absence of a correlation between the focused event and the candidate event. Based on the event data group acquired by the event data acquisition unit  12 , the time interval calculator  14  calculates a time interval between the occurrence date and time of the focused event and the occurrence date and time of each candidate event that has occurred within a predetermined time from the occurrence of the focused event. 
     The determination window generator  15  generates two or more determination windows W mutually differing in at least one of a duration and a position. The correlation determination unit  16  determines the presence or absence of a correlation between the focused event and the candidate event on the basis of the above points  1  to  4  by using the two or more determination windows W. The result output unit  17  outputs the determination result obtained by the correlation determination unit  16 . For example, the result output unit  17  stores in the related event storage  123  information on a candidate event (i.e., related event) determined to have a correlation with respect to the focused event. 
     The following describes a process executed by the correlation determination apparatus  10 .  FIG. 9  is a flowchart illustrating an example of a process executed by the correlation determination apparatus. 
     In step S 101 , the event selector  11  selects one type to be a focused event and one or more types to be candidate events from among two or more event types. The type of the focused event and the types of the candidate events may be input by a user. For example, it is assumed that a physician prescribing a certain medicine X for a patient A is a user. The user desires to know how administration of medicine X affects the patient A. In this case, for example, “take medicine X” is defined as a focused event, and “dizziness”, “heart rate rises to 20 bpm or more”, “body temperature rises to 38 degrees or more”, “systolic blood pressure rises to 140 mmHg or more” may be selected as candidate events. Step S 103  and subsequent steps are executed for all candidate events; however, one candidate event “dizziness” will be described as an example in the present embodiment, for the sake of convenience. In the following, simply referring to a “candidate event” indicates one candidate event to be processed among the selected candidate events. 
     Subsequently, the event data acquisition unit  12  acquires event data on the patient A, which is the event data on the focused event and the event data on the candidate event from the event data group stored in the event data storage  121 . The event data acquisition unit  12  generates respective time series data based on the event data extracted for each of the focused event and the candidate event (S 102 ). In the present embodiment, the data collected by the sensor attached to the patient A, the data input by the patient A or the physician (i.e., the user), etc. are stored in time series as event data on the patient A, and are stored in the event data storage  121 . 
       FIGS. 10A and 10B  are diagrams illustrating examples of respective time series data of focused events and a candidate event.  FIG. 10A  illustrates time series data of focused events. The time series data includes a case number and occurrence date and time for each focused event that has occurred. The case number is a number indicating the occurrence order of the focused event, and is assigned by the event data acquisition unit  12 , for example. 
     In addition,  FIG. 10B  illustrates time series data of candidate events. The time series data includes a case number and occurrence date and time for each candidate event that has occurred. The occurrence number is a number indicating the occurrence order of the candidate events, and is assigned by the event data acquisition unit  12 , for example. The respective time series data of the focused events and the candidate events are data indicating information similar to the information illustrated in  FIG. 1A . 
     Note that the case number and the occurrence number do not indicate the correspondence relationship between the focused event and the candidate event. Each of the case number and the occurrence number indicates the order of occurrence within a corresponding one of event groups. 
     Subsequently, the parameter acquisition unit  13  acquires parameters related to the determination window W from the parameter storage  122  (S 103 ). 
       FIG. 11  is a diagram illustrating a configuration example of a parameter storage. As illustrated in  FIG. 11 , the parameter storage  122  stores a candidate duration, a magnification of a shifting duration, a maximum time range, and the like. 
     The candidate duration is a candidate duration of the determination window W. The magnification of the shifting duration is a parameter for determining a shift duration of the determination window W. A value obtained by multiplying the duration of the determination window W by the magnification of the shifting duration is a shifting duration of the determination window W. Note that the shifting duration of the determination window W indicates an interval between the position of the determination window W before shifting the position of the determination window W and the position of the determination window W after shifting the position of the determination window W. The maximum time range is a parameter indicating an extraction range of the candidate event with respect to the focused event relating to each of the cases. For example, the maximum time range that may be considered to be affected by the focused event is specified. In the present embodiment, the unit of duration and of the maximum time range is seconds. 
     The candidate duration and the maximum time range may be set by utilizing prior knowledge in a case where there is a certain extent of prior knowledge such as empirical knowledge of the user (physician) (knowledge such as the effect of medicine X appearing within 1 to 2 hours after taking the medicine X). When the effect of medicine X appears within 1 to 2 hours after taking the medicine X, for example, a value indicating 2 hours may be set as the maximum time range. 
     Subsequently, the time interval calculator  14  calculates, for each of the focused events, a time interval (the absolute value of the difference in the occurrence date and time) from the occurrence date and time of the corresponding focused event to each of the candidate events occurring within the maximum time range (S 104 ). In the present embodiment, since the focused event (taking medicine X) is an event that occurs earlier than the candidate event (dizziness), “within the maximum time range from the occurrence date and time of the focused event” indicates a duration after the occurrence date and time of the focused event. In a case where the focused event indicates an event occurring after the candidate event, the maximum time range from the occurrence date and time of the focused event indicates a duration before the occurrence date and time of the focused event. Further, although the calculated time interval does not necessarily need to be an absolute value, in the present embodiment, an absolute value is used for the time interval in this embodiment in order to simplify the illustration (i.e., for convenience of illustration) by excluding a case where the time interval is negative. 
       FIG. 12  is a diagram illustrating an example of a calculation result of a time interval between a focused event and a candidate event.  FIG. 12  illustrates the calculation result of the time interval between the focused event and each of the candidate events occurring within the maximum time range from the occurrence date and time of the focused event, for each of the focused events illustrated in  FIG. 10A . In the present embodiment, the unit of time interval is seconds. Note that the information illustrated in  FIG. 12  corresponds to the information illustrated in  FIG. 1B . 
     In general, the maximum time range is much shorter than an occurrence interval of each of the focused events. For example, when the timing of taking medicine X is before bedtime (i.e., once a day) and the effect of medicine X appears within 3 hours, the time interval is calculated with respect to the candidate event in the three hours in step S 104 . Accordingly, it is unlikely that duplicated time intervals between the two or more focused events and the same candidate event are calculated. Note that when such duplication may occur, the time interval may be calculated by removing the duplication. 
     Subsequently, the determination window generator  15  generates a determination window W based on the parameter relating to the determination window W ( FIG. 11 ) (S 105 ). Specifically, for example, 0 is set as an initial value of a start point (the minimum value of the range of the determination window W) for each of the determination windows W having the four durations of 10, 20, 50, and 100, and the determination window W is generated as the start point is shifted by the shifting duration within the maximum time range. The end point of the determination window W (the maximum value of the range of the determination window W) is obtained by adding the duration of the determination window W (the size of the determination window W) to the start point of the determination window W. Further, the shifting duration is specified by multiplying the duration of the determination window W by the magnification of the shifting duration. As a result, two or more determination windows W differing from one another in at least one of the start point (minimum value) and the end point (maximum value) are generated. Note that the initial value at the start point is not necessarily 0. For example, insofar as the initial value is equal to or greater than 0 and equal to or less than the minimum value of the time interval calculated in step S 104 , the origin at the start point may be any value. 
       FIG. 13  is a diagram illustrating generation of a determination window. In  FIG. 13 , determination windows W having varying candidate event durations with respect to the start point are enumerated. Note that the determination windows W enumerated in  FIG. 13  do not strictly represent the determination windows W generated based on the parameters illustrated in  FIG. 11 . In the present embodiment, since the magnification of the shifting duration is 0.5 (i.e., less than 1), the determination windows W shifted by one duration from each other have a mutually overlapping range. 
     Note that in step S 105 , for each of the generated determination windows W, the start point and the end point and the like of the determination window W are specified.  FIG. 14  is a diagram illustrating an example of data specified by a generation process of a determination window. As illustrated in  FIG. 14 , in step S 105 , a determination window number, duration, a shifting duration, a start point, an end point, and the like are specified for each determination window W. 
     The determination window number is identification information of each determination window W. The duration and the shifting duration are duration and shifting duration used for generating the determination window W. The start point is a minimum value of a range of the determination window W. The end point is a maximum value of the range of the determination window W. Note that the duration and the shifting duration are information illustrated for reference, and may not necessarily be retained as the process result of step S 105 . This is because the shifting duration is not required in the process of the subsequent stage and the duration may be specified based on the start point and the end point. 
     Subsequently, the correlation determination unit  16  substitutes  1  for the variable k and substitutes the number of determination windows generated through the variable S (S 106 ). The variable k is a variable indicating the determination window number of the determination window W to be processed. Subsequently, the correlation determination unit  16  performs a determination process on the presence or absence of a correlation between the focused event and the candidate event with respect to the determination window W having the determination window number k (hereinafter referred to as “determination window Wk”) (S 107 ). The determination process is executed based on points  1  to  4  described above. There is no particular restriction on the order of the determination windows W to be processed. 
     Subsequently, the correlation determination unit  16  determines whether the value of the variable k is equal to or greater than S (S 108 ). That is, the correlation determination unit  16  determines whether step S 107  has been executed for all the determination windows W. When the value of the variable k is less than S (No in S 108 ), the correlation determination unit  16  adds 1 to the variable k (S 109 ) and repeats the step S 107 . When the value of the variable k is equal to or greater than S (Yes in S 108 ), the result output unit  17  outputs the determination result in step S 107  (S 110 ). 
     Subsequently, the details of step S 107  will be described.  FIG. 15  is a flowchart illustrating an example of a process of determining whether there is a correlation between a focused event and a candidate event with respect to one determination window. In the illustration of  FIG. 15 , the determination window Wk is the determination window Wk at the time when step S 107  of  FIG. 9  is executed. 
     In step S 201 , the correlation determination unit  16  executes the process related to point  2 . That is, the correlation determination unit  16  calculates, for each case, the rate Z1 in  of the number of time intervals in the determination window Wk to the duration of the determination window Wk and the rate Z1 out  of the number of time intervals outside the determination window Wk to the duration outside the determination window Wk. Note that the rate Z1 in  corresponds to the occurrence rate of candidate events within the determination window Wk. Likewise, the rate Z1 out  corresponds to the occurrence rate of candidate events outside the determination window Wk. The correlation determination unit  16  determines whether a ratio K1 of cases, for which the ratio R1=Z1 in /Z1 out  is equal to or greater than a threshold α, is equal to or greater than a threshold β. More specifically, in step S 201 , the correlation determination unit  16  executes the following process. 
     First, for each case, from among the time intervals calculated in relation to the case, the correlation determination unit  16  counts the number N(i) in  of the time intervals included in the determination window Wk, and the number N(i) out  of the time intervals not included in the determination window Wk. Note that i is a case number. Subsequently, the correlation determination unit  16  calculates Z1(i) in =N(i) in /W and Z1(i) out =N out /(W all −W) for each case. Note that W is the duration of the determination window Wk. W all  is the maximum time range. Subsequently, the correlation determination unit  16  calculates the ratio R1(i)=Z1(i) in /Z1(i) out  for each case. Subsequently, the correlation determination unit  16  determines whether the ratio K1 of the number of cases, where R1(i) is equal to or greater than the threshold α, with respect to the total number of cases is greater than or equal to β. 
       FIG. 16  is a diagram illustrating a calculation example of Z1 in  and Z1 out  and a ratio R1 for each case. In  FIG. 16 , W all  is 1000 seconds, and the duration of the determination window Wk is 100 seconds. Further, the total number of cases is 25. Further, in  FIG. 16 , the threshold α is 8 and the threshold β is 0.5. 
     For example, N(1) in =3, N(1) out =31 for a case 1 as a first example having a case number of 1. Accordingly, Z1(1) in =3/100=0.03, Z1(1) out =31/(1000−100)=0.034. Further, R1(1)=0.03/0.034=0.88. 0.88 is less than the threshold α. Accordingly, R1(1) related to the case 1 is determined to be less than the threshold α. Similarly, the determined results of the cases are illustrated in the rightmost columns of  FIG. 16 . If the number of cases determined to have the ratio R1 equal to or greater than the threshold α were to be 2, the ratio K1 of the number of cases (i.e., 2 cases) to the total number of cases (i.e., 25 cases) is 2/25=0.08. In this case, the ratio K1 is less than the threshold β. Hence, the determination in step S 202  is a negative result. 
     When the determination result in step S 202  is an affirmative result (Yes in S 201 ), the correlation determination unit  16  determines that the candidate event has a correlation with the focused event. 
     However, when the determination result in step S 201  is a negative result (No in S 201 ), the correlation determination unit  16  executes a process corresponding to point  4 . Specifically, the correlation determination unit  16  obtains a threshold T_TH corresponding to the start point of the determination window Wk (S 203 ). For example, the threshold T_TH may be obtained as follows. 
       FIG. 17  is a diagram illustrating an example of a method of obtaining a threshold T_TH according to a start point of a determination window. In a graph g 1  in  FIG. 17 , the horizontal axis corresponds to the start point of the determination window W, and the vertical axis corresponds to the threshold T_TH. 
     First, the correlation determination unit  16  assigns the minimum value among the candidate durations of the determination window W as a threshold HT 2  for the case where the start time is 0. In the present embodiment, candidate durations of the determination window W are 10, 20, 50, and 100 as illustrated in  FIG. 11 . Hence, the minimum value of the duration of the determination window W is 10. Thus, 10 is assigned to the case where the start time is 0. In this case, a point P 1  (0,10) is arranged in the graph g 1 . 
     Subsequently, the correlation determination unit  16  assigns the maximum value among the candidate durations as the threshold T_TH, with respect to the case where the start point is the maximum time range. Accordingly, 100 is assigned to the case where the start time is 1000. In this case, a point P 2  is arranged in the graph g 1 . 
     Subsequently, the correlation determination unit  16  linearly interpolates an interval between point P 1  and point P 2  to derive a straight line L 1 . 
     Finally, based on the straight line L 1 , the correlation determination unit  16  obtains a threshold T_TH corresponding to the start point of the determination window Wk. 
     According to the above method, the greater the value of the start time of the determination window W (i.e., the longer the occurrence time of the candidate event included in the determination window W is away from the occurrence time of the focused event), the greater the threshold T_TH may be. Note that the interpolation method between point P 1  and point P 2  is not limited to linear interpolation. For example, point P 1  and point P 2  may be connected by a curve. 
     Subsequently, the correlation determination unit  16  executes a process corresponding to point  3 . That is, the correlation determination unit  16  determines whether the duration of the determination window Wk is equal to or less than the threshold T_TH (S 204 ). When the duration of the determination window Wk exceeds the threshold T_TH (No in S 204 ), the correlation determination unit  16  ends the process of  FIG. 15 . The determination result in this case becomes undeterminable. 
     However, when the duration of the determination window Wk is equal to or less than the threshold T_TH (Yes in S 204 ), the correlation determination unit  16  executes the process corresponding to point  1 . That is, among the set of the time intervals calculated in step S 104  of  FIG. 9 , the correlation determination unit  16  calculates a ratio R2 of the occurrence rate of the time intervals included in the range of the determination window Wk to the occurrence rate of time intervals not included in the range of the determination window Wk (S 205 ). 
       FIG. 18  is a diagram illustrating an example of a method of calculating the ratio R2. In  FIG. 18 , the maximum time range W all  is 1000 seconds, and the duration of the determination window Wk is 20 seconds. The position (start point) of the determination window Wk is as illustrated in  FIG. 18 . The time interval between the candidate event and the focused event in each case is as plotted for each case in  FIG. 18 . 
     First, the correlation determination unit  16  counts the number N all  of the time intervals calculated in step S 104  of  FIG. 9 . In  FIG. 18 , N an =33. In addition, the correlation determination unit  16  counts the number N of time intervals included within the range of the determination window Wk among the set of N all  time intervals. In  FIG. 18 , N=5. 
     Subsequently, the correlation determination unit  16  calculates a rate Z in  of N with respect to the duration W that is the size of the range of the determination window Wk. In  FIG. 18 , Z in =5/20=0.25. Subsequently, the correlation determination unit  16  calculates a rate Z out  of the number of time intervals outside the determination window Wk to the time excluding the duration of the determination window Wk from the maximum time range W all . In  FIG. 18 , Z out =(N all −N)/(W all −W)=0.028. Subsequently, the correlation determination unit  16  calculates a ratio R2 of Z in  with respect to Z out . In  FIG. 18 , R2=Z in /Z out =8.93. 
     The following description is given by referring back to  FIG. 15 . Following step S 205 , the correlation determination unit  16  determines whether the ratio R2 calculated in step S 205  is equal to or greater than a threshold R_TH (S 206 ). The threshold R_TH is set in advance. For example, with reference to  FIG. 18 , when the threshold R_TH is 8, the ratio R2 is greater than or equal to the threshold R_TH. Accordingly, in this case (Yes in S 206 ), the correlation determination unit  16  determines that the candidate event has a correlation with the focused event (S 207 ). However, when the ratio R2 is less than the threshold R_TH (No in S 206 ), the correlation determination unit  16  determines that the candidate event does not correlate with the focused event (S 208 ). 
     Note that in step S 110  of  FIG. 9 , for cases of the correlation determination unit  16  determining a correlation with any of the determination windows Wk, information indicating a focused event, information indicating a candidate event (related event) determined to have a correlation with the focused event, and the start point and the end point of the determination window Wk may be stored in the related event storage  123 . 
       FIG. 19  is a diagram illustrating a configuration example of a related event storage. In  FIG. 19 , the related event storage  123  stores information indicating a focused event, information indicating a related event, and a high correlation period in association with one another. The high correlation period is information indicating the start point and the end point of the determination window Wk determined to have a high correlation with respect to the focused event and the related event. 
     When the correlation determination unit  16  determines that there is a correlation with respect to two or more determination windows Wk, the start point and the end point of the determination window Wk having the highest ratio R2 may be output. Alternatively, based on the ratio R, the start point and the end point of each determination window Wk may be sorted and output. When the correlation determination unit  16  determines that there is no correlation with respect to all the determination windows Wk, information indicating that there is no correlation between the focused event and the candidate event may be output. 
     In the above description, when the correlation determination unit  16  determines that there is a correlation with respect to at least one determination window Wk, there is a correlation between the focused event and the candidate event. However, as a requirement for recognizing the presence of a correlation between the focused event and the candidate event, there may be an additional requirement that the number of determination windows Wk determined to have the correlation does not exceed a predetermined threshold γ. In this case, information (start point and end point, etc.) on all the determination windows Wk determined to have correlation may be output. However, when there are two or more determination windows Wk having overlapping ranges, for example, only information on the determination window Wk in which R2 or K1 is the maximum may be output. 
     When the number of the determination windows Wk determined to have the correlation exceeds the threshold γ, values such as R_TH, α, β, and the like, which are thresholds for determining the presence or absence of a correlation, may be reviewed. For example, each threshold may be set by a method as described below. After the review of the threshold, steps subsequent to step S 106  in  FIG. 9  may be executed. 
     An example of a method of determining the threshold R_TH and the threshold α will be described. With respect to the illustration of  FIG. 18 , the following formula (1) is established with respect to the relationship between Z in /Z out  and the threshold R_TH. 
     
       
         
           
             
               
                 
                   
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     In the formula (1), the left side corresponds to Z in . The inside of the parentheses on the right side corresponds to Z out . 
     With respect to the illustration of  FIG. 16 , the following formula (2) is established for the relationship between Z1(i) in /Z1(i) out  and the threshold α. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     Basically, from the relationship between the formula (1) and the formula (2), the same value set for the threshold R_TH may be set for the threshold α. Then, the threshold R_TH may be determined based on the ratio R2 calculated using the known data, for example, on two event types that are known to have clear correlation. 
     For example, it is assumed that it is known that the focused event X and the related event Y have a correlation. Further, it is assumed that the relevant time range is also known, and it is known that the probability of occurrence of the related event Y 10 to 20 minutes before the focused event occurs is high. 
     Under such circumstances, as described above, using the known event data of the focused event X and the related event Y, a time interval is calculated, for each case, between the focused event X related to the case and each of the related events Y within the maximum time range from the focused event X. 
     Subsequently, the occurrence rate Z in  of the related event in the determination window W and the occurrence rate Z out  of the related event outside the determination window W are obtained using the determination window W related to the relevant time range (10 to 20 minutes beforehand), and then the ratio R2=Z in /Z out  is obtained. 
     Since the value of R_TH is the lowest threshold, the value of R_TH may be set to have at least the value of the ratio R2 or more. Note that in the determination of the threshold value R_TH, R2(p) may be calculated for each person p using known data of two or more persons, and the maximum value of R2(p) may be set as R2. 
     As described above, according to the present embodiment, for each of the determination windows W, the occurrence rate of the candidate event in the determination window W is compared with the occurrence rate of the candidate events outside the determination window W, and the presence or absence of a correlation between the focused event and the candidate event is determined. Accordingly, the accuracy in extracting events having a correlation with a certain event will be much improved, compared to the case where the presence or absence of a correlation between the focused event and the candidate event is simply determined based on the frequency or the like. 
     Note that this embodiment may be applied to events (events) other than events related to daily life of a person. For example, the embodiment may be applied to an event indicating a state of an apparatus. In addition, the embodiment may be applied to an event (log information) output from a computer system. 
     In the present embodiment, the focused event is an example of a first type event. The candidate event is an example of a second type event. The time interval calculator  14  is an example of a calculator. The correlation determination unit  16  is an example of a determination unit. 
     The embodiments of the present invention have been described in detail above; however, the present invention is not limited to a specific one of the embodiments, and various modifications and changes may be made within the scope described in the claims. Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes may be made within the scope of the gist of the invention described in the claims. The threshold value α is an example of the first threshold value. The threshold value β is an example of the second threshold value. The threshold value T_TH is an example of the third threshold value. 
     According to one aspect of the disclosure, the extraction accuracy of an event having a correlation with a certain event may be improved. 
     All examples and conditional language provided herein are intended for illustrative purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.