Estimating Parameters in Mathematical Model for Societal Booms through Bayesian Inference Approach

In this study, based on our previous study, we examined the mathematical properties, especially the stability of the equilibrium for our proposed mathematical model. By means of the results of the stability in this study, we also used actual data representing transient booms and resurgent booms, and conducted parameter estimation for our proposed model using Bayesian inference. In addition, we conducted a model fitting to five actual data. By this study, we reconfirmed that we can express the resurgences or minute vibrations of actual data by means of our proposed model.


Introduction
Booms emerge in many fields and are closely tied to our everyday life. For example, a fashion in clothing, makeup, sports, a movie and food(we call "societal booms"). These examples show that "interesting information" about the individual boom passed at a rapid rate to a large number of people in a short period of time. Furthermore, in a sense, we can regard an infectious disease as a boom, like influenza or SARS(we call "epidemiological booms"), which is infected by viruses that are transmitted from person to person. In the above examples, it is the most important things that both "interesting information" and "viruses" are transmitted by some form of contact. Hence, we considered that a spread with an interest in products, movie, food, etc resemble the transmission dynamics of viruses in ways.
Studies on epidemiological booms that employed mathematical models were founded on differential equations in the late 19th century, and mathematical modeling using differential equations was developed by Kermack and McKendrick in the 1920s. This field of study gained attention among researchers owing to the spread of emerging infectious diseases, such as AIDS in the 1980s, that posed risks in developed countries. This research continues to progress ( [1], [3], [9]). On the other hand, there are many studies that researched societal booms from a sociological or psychological perspective, but few were conducted from a mathematical point of view. However, in recent years, companies have concentrated their marketing efforts on the development of hit products that emphasize customer taste and trend analysis by using social networking services (SNS) such as Twitter and Facebook.

Mathematical Model
In this section, we explain a mathematical model for societal booms which was derived in [16].

Three key points to derive our proposed model
Here, we explain the three key points which were discussed to derive our mathematical model. The first point is the contact. Infectious disease epidemics such as influenza are thought to occur when a virus invades and infects a healthy person's body from contact with an infected person. In our proposed model, we define "interesting information" to be a "virus" , which is transmitted from people in an on-boom state to those whom the boom has not reached yet (pre-boom). Thus, our proposed model incorporates the perspective of the contact, which was not considered in [14].
The second point is the time delay. The Diffusion of Innovation theory, developed by E.M. Rogers, separates consumers into five categories based on the speed at which people are likely to adopt innovation (innovators, early adopters, early majority, late majority, and laggards). Based on this theory, we think that time lags exist in the adoption of booms by people in a social system, and thus developed a model that considers the effects of a time delay. Hutchinson [6] suggested the following logistic equation with time delay τ . This equation shows that the solution does not fluctuate monotonously but exhibits complex behavior such as oscillatory behavior depending on the magnitude of the time delay: Furthermore, the biologist R.M. May [11] regarded (1) as a mathematical model that expresses the temporal changes of the herbivorous animal population x(t), and asserted that the biological definition of a time delay was "the time required for the regeneration of plants that is suitable for animals to eat". Additionally, May received acclaim for fitting results from an experiment on the Australian sheep blowfly (Lucilia cuprina) conducted by Nicholson [15]. Based on these experimental results, we regarded the definition of a time delay for societal booms as "the time required for a boom adopter associated with contact and resurgence to pick up a boom and take action", and incorporated the concept of the time delay into the derivation of a mathematical model.
The last point is the existence of influencer and "Sakura 1 ". In the current landscape, companies actively employ influencer and "Sakura" in their marketing strategies. Our proposed model expresses their presence by depicting a resurgence caused by opinion leaders and forced changes in the number of people.
As described above, our proposed model is a natural extension of the boom model developed in [14] and is derived from the above three perspectives. We expect that the model will be able to capture various types of boom data.

Mathematical model for societal booms
In [16], we proposed a new mathematical model to explain societal booms based on the background described above.
Here, according to [16] we derive the mathematical model for societal booms.
First, we assume the state of the boom participants at any given time to be one of the following: • State 1 pre-boom: Condition where there is a potential to adopt a boom • State 2 on-boom: Condition where the boom is captured • State 3 rooted boom: Condition where the boom is retained • State 4 unrooted boom: Condition where the boom did not take off Furthermore, at a given time t, we assume the number of boom participants in each state to be y 1 (t), y 2 (t), y 3 (t), and y 4 (t) respectively, for States 1 − 4. Then we represent the changes of a customer's state by using the following equations: Here, the variables α, β, γ, δ, ε, ζ and τ 1 , τ 2 in (2) respectively represent the rate of transmission ("infection") of the boom among people in a pre-boom state per unit time, rate of retention among people in an on-boom state, rate of people who quit the boom, adoption rate of the boom by people in a pre-boom state, rate of resurgence from rooted boom to pre-rooted state, and the production rate of people in an rooted boom state. τ 1 and τ 2 are parameters that show the time delay, and τ 1 < τ 2 . Here, α > 0, β + γ > 0, δ > 0. In particular, αy 2 (t − τ 1 ) is called as infectivity and is an important indicator that characterizes the boom model.
In this model, the rate at which people go from a pre-boom state to an on-boom state is proportional to the number of people in a pre-boom state and the number of people who changed to an on-boom state before τ 1 (the first term of the first equation in (2)), and we assume that people in a pre-boom state naturally adopt booms at a fixed rate (the second term of the first equation in (2)). Moreover, it expresses the resurgence of the boom by transferring the rate of people who became on-boom prior to τ 2 (the third term of the first equation in (2)). In addition, ζ expresses the ratio of people in an rooted boom state who were compelled to enter that state (in other words, "Sakura"). Furthermore, the given ratio of people in an on-boom state enter an rooted or unrooted state(the second and third term of the second equation in (2)).

Stability of the equilibrium point
In this section, we analyze the stability of equilibrium by investigating location of the roots of associated characteristic equation. Here, since the right side of each equation in system (2) consists of only y 1 , y 2 , we study the stability of the equilibrium point for the first equation and the second equation in system (2). The first equation and the second equation in system (2) possess a trivial equilibrium E 0 = (0, 0), if ζ = 0, and non trivial equilibrium In this study, we analyze the stability of non trivial equilibrium E 1 .
First, upon the following change of functions and substitutions Since y * 1 , y * 2 are the equilibrium point, (3) becomes the following form: Regarding this equation as system ODE with the equilibrium point (0, 0), we have d dt where Therefore, the characteristics equation of the above system ODE takes the following form: which implies Theorem 3.1 We consider Esq. (2), where α, β, γ, δ, and τ 1 , τ 2 are non-negative numbers and ε, ζ are real numbers. And let us assume that moreover, or Then we shall show that every root of the characteristic equation (5) must have negative real part. proof 3.1 Suppose for contradiction that λ = µ + iω is a root of (5) with µ ≥ 0, ω = 0. Then, At first, from µ > 0 and (7), we have Next, From (8), (9) or (10), (11), we have This is contradiction shows that every λ has negative real part. Hence every solution of Esq. (2) tends to zero exponentially as t → ∞.

MCMC methods
In Bayesian inference approach, the complicated and intractable probabilistic models can be estimated by numerical sampling methods such as MCMC, which has been widely applied in recent years. The details of MCMC methods are given in Robert and Casella [18].
Monte Carlo simulation generates pseudo-random numbers for exploring posterior distributions. In the MCMC algorithm, the pseudo-random number is a Markov chain. The MCMC algorithm exploits the property of a Markov chain to generate pseudo-random numbers from a posterior distribution, even for a complicated model. It first constructs an ergodic Markov chain with a stationary distribution equaling the target distribution. By iterating the Markov chain transitions from suitable initial value, it eventually obtains the target distribution.
In this paper, we employs a typical MCMC algorithm called the Metropolis-Hastings (M-H) algorithm (see Metropolis et al. [13]; Hastings [5]). The M-H Algorithm given below builds its Markov chain by accepting or rejecting samples extracted from a proposed distribution. This algorithm is generally used in Bayesian inference approach (cf. [8]).
By running the M-H algorithm, we can sample the distribution f (θ|Y ), and usually the mean value of θ j , after a given burn-in time j * .

Using Real Data to Evaluate Validity of Proposed Model
Parameter Estimation Steps(PES) below is the procedure for the parameter estimation for the model fitting to actual data conducted in this study. The values referred in Step 2 of PES were determined from actual data as follows: Parameter Estimation Steps(PES) • Step1: Set appropriate initial values for α ∼ ε.
• Step4: Visually confirm the fitting to actual data, and confirm the coefficients of determination.
• Step6: Repeat steps 3-5 to determine the final parameters based on the sufficient condition in Theorem 3.1, changes in the coefficients of determination and visual observation.
We determined the values for time delay parameters τ 1 and τ 2 from the actual data, referencing events that occurred prior to each cultural phenomenon. In particular, for τ 1 we observed the first peak and set the initial value. Moreover, for τ 2 we set the initial value to be the duration between this point of change to a large peak, on unusual changes such as peaks created from resurgence.

Coefficient of determination
In this study, we introduce the coefficient of determination R 2 as an index, which indicates the percentage change between the solution of the proposed model and an actual data, as follows: Then, from the following form (12), the range is from 0 to 1 and the R 2 of 1 means an actual data can be predicted without error from the solution of the proposed model. Here,Ȳ is a mean of an actual data, and E i = Y i − F θ i ,Ēis a mean of E i . In this section, according to [16], we compare the solution of our proposed model to the solutions of the proposed model in [14](Prior Study1) and [21](Prior Study2), to verify the superiority of our proposed model which is used in this study. We employed the number of tweets related to a nuclear power plant as the basis for comparison, in Prior Study2 used this data to compare their study with Prior Study2. Here, the parameter values were estimated according to Algorithm 1 above. Figure 1 also shows that the model developed in Prior Study1(dashed line) does not accurately express the resurgence following the decline. The model developed in Prior Study2(dashed-dotted line) captures the resurgence following the decline but could not reproduce the second peak or the dip in the middle stage. On the other hand, our proposed model(solid line) is able to capture the resurgence following the decline, the second peak, and the midway dip that were not expressed by the models in Prior Studies1 and 2. The accuracy of the model is not only visually evident but can also be seen from the value of the coefficient of determination. The figure is R 2 = 0.9609 for our proposed model compared to R 2 = 0.9129 in Prior Study1 and R 2 = 0.9238 in Prior Study2. Therefore, it can be said that there is high utility in using our proposed model to fit the actual data, as we show below.

La La Land
La La Land is a buzz-worthy American romantic musical film. La La Land premiered at the 73rd Venice International Film Festival on August 31, 2016, and was released in the United States on December 9, 2016 and has grossed $446 million to data. Moreover it received 14 Academy Award nominations at the 89th Academy Awards and won in six categories, including Best Director and Best Actress. From the above points, we regards the film "La La Land" to be appropriate for reflecting a transitory boom and used the data as the subject for testing.
In this study, we used the number of user reviews left on Yahoo!Japan Movies during the period from August 2016 to July 2018, including March 2017 when La La Land won in six categories at the 89th Academy with respect to the fit proposed model to actual data.
The actual data in Figure 2 show a decline after the first peak on March 2017 and finally decrease after two small peaks. The graph from the proposed model shows a high degree of fit to the first peak. In addition, we observe that it is able to largely replicate the subsequent curves including the leveling-off and decrease rapidly towards the end of the period after two small peaks. In addition, the coefficient of determination, which is the measure of how well a model explains the data, shows a high value at R 2 = 0.9688; this reveals that the proposed model has a high degree of fitness. On the other hand, we observe that fitting to two small peaks in the middle of the period is not good.

"Ten-nensui (Pure Water)"
Suntory "Ten-nensui" is best-selling mineral water which made with water from renowned water resources in Japan, including the Minami-Alps. All Suntory Ten-nensui products are made from "soft water", clear in color, and beloved for their refreshing taste.
In this study, we used Twitter data from before and after the product launch on 4/17/2019 to test the effectiveness of the "GREEN TEA CAMPAIGN" which is the new product of "Ten-nensui (Pure water)" with respect to the fit proposed model to actual data.
The actual data in Figure 4 show a decline after the first peak. After that, it has a small second peak once more, but it decreases again. Similar to the other examples, the proposed model shows a high degree of fit to the first peak. In addition, we observe that the graph from the proposed model is able to well reconstruct the processes of declining and even expresses the small decline that occurs from 4/18/2019 to 4/21/2019. The high degree of accuracy in the fitness is evident from the large coefficient of determination, R 2 = 0.9612. On the other hand, we observe that the graph from the proposed model can't express well the second peak in the middle of the period.

Honkirin Beer
Honkirin Beer is a Happoshu (low-malt) beer that was introduced on March 13, 2018. In a cost-conscious environment, Honkirin Beer became the biggest hit among new beer releases in FY 2018 owing to its high quality and low price. According to the brewer Kirin, Honkirin Beer underwent a product renewal in mid-January 2019 for an even more refined authentic taste. As a result, the product logged a record sales volume in February (1.12 million cases), second only to its release in March 2018 (1.17 million cases).
Here, too, we used Twitter data from before and after the product launch on 4/22/2019 to test the effectiveness of the Honkirin Beer campaign with respect to the fit of the proposed model to actual data.
The actual data in Figure 6 show a decline after the first peak slowly with one peak and one bottom on April 26 and 28 2019. Similar to the other examples, the graph from the proposed model shows a high degree of fit to the first peak. In addition, we observe that it is able to fit the subsequent curves to the actual data except for two specific points. The high degree of accuracy in the fitness is evident from the large coefficient of determination, R 2 = 0.9365. On the other hand, we observe that the graph from the proposed model isn't able to express well two specific points. Future challenges include improvements to these points.

Cup-noodle
The first instant noodles were invented by Momofuku Ando, who later founded the well-known food company Nissin Food, in Japan in Osaka in 1958. Currently, the instant noodles accomplish evolution of various form and are mainly sold as Cup-noodle at convenience stores and supermarkets. In Japan, Cup-noodle is the most convenient quick meal and from the busy Japanese businessmen and women to the school children, it seems everyone enjoys them.
Here, we used Twitter data from 4/10/2019 to 5/3/2019 before and after the product presentation on 4/23/2019 to test the effectiveness of the cup-noodle presentation with respect to the fit of the proposed model to actual data.
The actual data in Figure 8 show a decline after the first peak, and several vibrations and a sharp increase again towards the end day of this period. Similar to the other examples, the graph from the proposed model shows a high degree of fit to the first peak. The high degree of accuracy in the fitness is evident from the large coefficient of determination, R 2 = 0.9354. In particular, the graph of the proposed model expresses the rapid increase at the end day of this period. These points are advantages of our proposed model. In Japan, there are both the Christian era and the Japanese imperial era names are used, and when we have new emperor, new era starts. There was a big event in Japanese royal family this year. Current Emperor Akihito abdicated the throne on April 30 and his son, Crown Prince Naruhito, enthroned the throne on May 1. New Imperial era "Reiwa (Beautiful Harmony)" is taken from a verse in the oldest anthology of poems called Manyo-shu. Here, we used Twitter data after the name of new Imperial era was announced on April 1 to test public interest of new Imperial era "Reiwa" for the fit of the proposed model to actual data.
The actual data in Figure 10 show a rapid, sharp spike followed by an easing, which is a characteristic pattern of transitory booms. The graph from the proposed model fits the actual data with a very high overall accuracy. The high degree of accuracy in the fitness is evident from the value of the coefficient of determination, R 2 = 0.9910. In particular, the solution from the proposed model even expresses the small decline that occurs from 4/4/2019 to 4/6/2019. On the other hand, from the graph of the solution to the proposed model, we can see interests of Twitter users of new Imperial era "Reiwa" towards the date when the Era name changes from "Heisei" to "Reiwa". Thus, we believe that an advantage of the proposed model is its ability to express vibrations by incorporating time delays and "Sakura" data.

Conclusion
Based on our previous study, we examined the mathematical properties, especially the stability of the equilibrium for our proposed mathematical model in our previous study. By means of the results of the stability in this study, we also used actual data representing transient booms and resurgent booms, and conducted parameter estimation for the proposed model using Bayesian inference. In addition, we conducted a model fitting to actual data. By this study, we reconfirmed that we can express the resurgences or minute vibrations of actual data by means of our proposed model. However, some issues remain. The first one is the validity of the proposed model. This study and our previous study only tested a total of 10 cases. In our future work, it will be necessary to take various examples of boom data, test the validity of our proposed model, and address the relationship between the actual data and parameters. Additionally, we would like to conduct an analysis using data not just for domestic booms but also global booms to test the validity of our proposed model. In addition, there is room for improvement in the parameter estimation method. In this study, we performed parameter estimation using Bayesian inference (the MCMC-MH method). In particular, there was a slight divergence in the second half of the graph and between the vibrations of the real data (actual vibration) and the vibrations of the mathematical model solution, as evinced by the fit to the resurgent boom data. This is considered to be a limitation in the model's reproducibility, but on the other hand, we will need to conduct testing in order to include improvements in the Bayesian inference method. In addition, there is room for improvement in how to determine τ 1 and τ 2 , which represent time delays, and ζ, which represents the presence of "Sakura". In our future work, we would like to search for parameters that have practical significance by incorporating findings from marketing research during the parameter estimation. The expansion of the model is also an important topic. In particular, a mathematical model should be developed that incorporates probabilistic items to handle unexpected events. The "Sakura" parameter is a unique point of the proposed model, but realistically, a model is needed for scenarios other than that with a constant presence of "Sakura". We also must consider models with functions that have time-dependent parameters and timedependent "Sakura".
On the other hand, we must also advance mathematical analysis of areas such as the asymptotic behavior of mathematical models and their behavior around the points of equilibrium. In this study, we derived a sufficient condition with respect to the stability of the equilibrium for our proposed model, but it is certainly not necessary. In fact, the parameters to fit the actual data, "Reiwa" and "Cup-noodle" , didn't satisfy this sufficient condition. However, fitting to the actual data is superior to several prior studies, and it is thought that our proposed model is superior for the means to express vibration and resurgence of the actual data during a certain period of time. In our future challenges, we improve the method of estimating parameters, by clarifying an association between mathematical properties and a Bayesian inference approach.
In recent years, the development of technology has made it relatively easy to obtain large data (both quantitatively and qualitatively), but a significant challenge has been how to use this large amount of data. We would like to continue research efforts in anticipation of the practical contribution of the mathematical model and parameter estimation method using a Bayesian inference approach in this study to corporate marketing strategies.