Investigate the Application of the AIDA Model in the Sport Industry: The Effectiveness of Advertisements of Famous Athletes on Students' Attitudes

Introduction

There are many companies and organizations around the world that intend to differentiate their brand and logo from other competitors by using different strategies, including targeted advertising so that they can compete in different markets (Esmailpour et al., 2017). Advertisement is one of the most important components of promotion that can play an important role in changing the mentality of customers towards goods or services, communicating with consumers, and introducing new products in marketing strategies (Hasani et al., 2019). Commercial companies are usually looking for the best arena to advertise their products, and among them, sports are one of the areas that have provided the most effective advertising spaces to customers and target audiences (Ojagh et al., 2017); so that the growth of the sports industry and the simultaneous increase in the amount of advertising shows that companies have realized the power of advertising through sports to achieve different goals (Peguraro et al., 2010).

Just as Pyun and James (2009) acknowledged, advertisements through sports can influence the purchasing behavior of fans and shape consumers' beliefs and attitudes towards advertisements more positively and helpfully. In general, a positive attitude toward advertisements is referred to as an "experienced attitude" that can be a suitable response to favorable or unfavorable advertisements and affect purchasing behavior (James., 2011). Advertising through sports is an important means for many companies to influence their attitude in a positive direction due to its flexibility, popularity, and wider audience reach (Kim & Hobb., 2016). Considering the positive and influential features of the sports industry, companies and organizations use different advertisement strategies and approaches, one of which is the use of famous athletes in advertising or endorsement (Brigham., 2011). An endorser is a person who is known to the public for his success in fields outside the field of the endorsed product or service (Gholi-Aligudarzi., 2017). Champions, athletes, and cinema and theater actors who have this feature commit for a long time to support the desired brand based on a specific financial contract (Bashiri et al., 2019). This has caused companies and organizations to make extensive changes by using famous personalities in the process of branding and advertising (Rasouli et al., 2015). Therefore, endorsing products and services using famous athletes can create a positive attitude in the minds of consumers concerning the desired brand and increase their willingness to buy (Flint et al., 2015); such that the researchers stated; an athlete is not a person who is just training or competing, but can appear in the role of a social influencer and influence the lifestyle of the general public through his internal and external factors (Hodge & Walker., 2015). That’s why companies in the modern age tend to use sports stars to endorse their services and products, because they are more present in the media than other famous people, and therefore, have various capabilities such as their extraordinary performance on the field, attractive appearance and different lifestyle, which seems attractive and exciting to the majority of people in the society (Eidi et al., 2015).

In addition, any athlete who has these capabilities can be used in advertisements even in retirement (Constanzo & Goodnight., 2005). In this advertising method, famous athletes can appear in four possible ways; the clear and obvious state that the athlete himself endorses the product directly; the implied state that the athlete shows he is the real consumer of the product; the commanding mode, in which the athlete suggests the product, and simultaneous mode, in which the athlete allows his image to be attached to a product (Mahmoudi et al., 2017). When the athlete is in one of these four modes to promote a product or service, it establishes a strong connection with consumers' feelings and experiences, and over time, the product and the endorser become a part of the consumer's memory (Mahmoudi et al., 2018) which in turn strengthens the attitude of consumers. On the other hand, famous athletes, from the endorsement point of view, persuade the audience, creating a favorable feeling and confidence, causing awareness and strengthening the attitude of the fans, and leading them to purchase (Mukherjee., 2009). Noting the effectiveness of advertising through famous athletes is very important because it must be determined to what extent the advertising for which a lot of money has been spent can meet the needs of the company to achieve the goals it has set. And also, how much has it been able to help the company achieve its overall marketing goals? (Hasanqholipour & Mousavi., 2012). Evaluation of the advertisement effectiveness by famous athletes will let companies, institutions, and natural and juridical persons know if there is no return on investment in advertising activities and if the goals and results will be neglected (Ojagh et al., 2017). Therefore, the effectiveness of advertisements of famous athletes, like other advertising activities, requires objective, measurable, and evaluable results, and it should be taken into consideration (Mohammadian & Hosseini., 2010).

Previous studies also show the effectiveness of famous athletes in advertising in some way. The first example is a study by Jiang et al. (2022), which shows that famous athletes have a close and respectful relationship with their followers and can also easily attract new fans which has been noticed by many commercial companies. For instance, Zhang Ji-Ki, a prominent table tennis player and gold medalist of the Men's World Championships, through polite and respectful communication with his one million and five hundred thousand fans has been able to attract many financial sponsors to carry out advertising activities on his Instagram page, because the outstanding performance of this famous player has led to the strengthening of the attitude of the fans. The study by Jansen et al. (2022) also showed that famous athletes who are present in both the real world and the virtual world and perform better than other athletes, are more successful in creating a positive attitude to purchase products and also acknowledged athletes who perform well but are relatively famous can be effective in advertising when there is a proportion and consistency between the athlete and the advertised product.

Bagheri and Biglari (2020) also indicated that commercial companies can have a direct effect on their sales by strengthening the beliefs and attitudes of consumers and stated that product and service endorsement by famous athletes is one of the best strategies to change people's attitudes and credibility. Also, in a study focused on the attitude of black and white people towards Nike ads, Interavia et al. (2020) showed that black people are twice as likely to see Nike ads performed by famous athletes in all countries most of the time because they believed that advertising through famous athletes has more credibility and affects their purchase intention. Aririgase et al. (2019) also stated in their study that there is a direct and significant relationship between the endorsement of famous athletes and purchase intention. Bashiri et al. (2019) showed that popularity, attractiveness, expertise, similarity with the product, personality, and trustworthiness are very effective in the acceptance of advertisements of famous athletes by consumers, and commercial companies should note these factors to create purchase intention.

In addition, Zhang and Liu (2019) stated in their study that the race and ethnicity of famous athletes are effective in advertising and creating a positive attitude in customers. Also they showed that in China, Chinese athletes are more effective in advertising than dual citizenship athletes. On the other hand, Ruihali et al. (2010) showed in research that it is not always possible to expect a positive effect from the endorsement of famous athletes. They stated that advertisements can be effective when there is consistency and accordance between the athlete and the advertised product, and if this consistency is not established, the effectiveness of advertisements cannot be expected. For this reason, it can be seen that the ads of Speedo Company by the famous American swimmer Michael Phelps, Roger Federer's ads for Nike, Clear Health company's ads by Christian Ronaldo, and many other examples are effective due to the existing accordance. Along with the growth of endorsement of famous athletes in the world, this topic has also grown in Iran. As Sepahrian et al. (2019) stated; Today, Iran's famous sports personalities are more active in endorsements than ever before, and many organizations and companies want to display their sports or non-sports goods through famous athletes. Also, Esmaili et al. (2019) state that if the principle of proportionality between the product and the athlete in domestic advertising is observed, the advertisement will not only be more targeted, but also increase the trust of the product and the reputation of the famous athlete in the Iranian society. In this regard, we can mention the effective advertising of Alireza Jahanbakhsh, a famous Iranian footballer in the clothing industry, who was able to succeed in this field due to his attractive appearance and high compatibility with the product. Finally, according to the latest report of the world of economy newspaper, famous Iranian athletes have been able to grow in this field, so that the reports show that athletes such as Seyed Mehdi Rahmati and Mehdi Mahdavi are the two nostalgic football players of Iran in the advertisements of Mihan, Mehdi Taremi, and the exchange advertisements Binance, Karim Bagheri in the advertisements of Banser and Elahe Mansoorian in Wushu, have been able to do effective advertisements by advertising the sale of property in the form of a validator (World economy newspaper., 2017).

The contents above show the importance of the effectiveness of advertisements, so studies show that there are four criteria for evaluating the effectiveness of advertisements; First, every advertising effort must have a clear and measurable goal. Second, measuring effectiveness should establish an indirect relationship between the expected impact of advertising and the ultimate marketing goals. Thirdly, the criteria should represent the causes of the creative idea resulting from the advertising campaign, and fourthly, the effectiveness of the advertisement should identify the long-term effects on the brand name and explain the reason for it (Kordloo et al., 2015). In the same spectrum, there are different methods to evaluate the effectiveness of advertisements in the four mentioned criteria. Communication tests, focus groups, psychological or physiological tests, frame-by-frame, and in-market tests can be mentioned. Also, some hierarchy of effects models is used to measure effectiveness, among which Dagmar, AIDA, Lavage, and Steiner and combined technique models can be mentioned. The DAGMAR model was first introduced into the advertising literature by a person named Cooley in 1961. DAGMAR stands for "Advertising Objective Definition for Measured Advertising Results". Based on this model, advertising managers can have a set of hierarchical advertising objectives instead of focusing only on sales objectives (Ojagh et al., 2017). The Hierarchy of Effects model or Hierarchical Consequences of Effectiveness was proposed in the early 1960s by Lavidge and Steiner. This model assumes that consumers who buy a product move through a sequential hierarchy from awareness to product purchase (Keshmiri et al., 2016). But the AIDA model, which is one of the advertising effectiveness models, has four stages, the components of which include attracting attention, creating awareness and interest, creating desire, and making a purchase. Based on this model, in order for advertising to lead to success, it should be designed in such a way that all the four mentioned steps, which are equal in importance, go through their time process, while the AIDA model was proposed in 1898 by Elmo Lewis and in the year Its model was designed by Strong in 1925 (Dolatyari & Porsoltani-zarandi., 2016). Researchers believe that the AIDA model is one of the most appropriate methods to determine the effectiveness of any type of advertising or any type of activity that requires these four stages (Ojagh et al., 2017). Previous studies also show that the AIDA model is used in different axes due to its high application, so in a study conducted by Mohammadi and Esmaili (2012), it was found that the use of the AIDA model in television advertisements is more effective. Compared to other models, it creates customers in all four steps. Also, Lee and Hoffman (2015) showed in their study that teaching the four steps of the AIDA model to marketing professionals and advertising industry activists will not only improve their business information, but will also cause enthusiasm and gain new experiences that will improve the knowledge of marketers. In another study conducted by Moghaddis and Farzan (2017) on the effectiveness of the animation of football players in attracting children to football using the AIDA model, they stated that the animation of football players strengthens children's attention to football, in the interest of children to football, and has been effective in children's desire to play football and finally in playing football by children. Considering that there are many studies in the field of attitude towards advertisements through sports, fewer studies have been done in explaining the AIDA model by considering the effectiveness of advertisement and the attitude of the fans. Therefore, conducting such research that includes all famous Iranian athletes in the first place and secondly, creating a clearer horizon for future research seems necessary to provide a field of validation for younger athletes. Considering that the current research community includes students who are fans of famous athletes, it seems that they would express more appropriate and scientific opinions, which is important. Therefore, the current research seeks to answer the question of whether famous athletes can influence people's attitudes through the four steps of the AIDA model.

Figure (1): The conceptual model of the research

Research Methods

The method of the current research is descriptive-survey and according to the purpose, it is a particular type of applied studies. The statistical population of the present study consisted of students studying in physical education colleges in Tehran. The number of the target sample in this study based on Morgan's table was considered to be 384 people. It is worth mentioning that the sampling method was voluntary and available, and the criteria for selecting the samples was to see at least three advertisements from famous athletes, which were asked before completing the questionnaire and in the demographic questions. The statistical sample of the research completed the questionnaire electronically. So that the researchers distributed the questionnaire by creating a link to the questionnaire and sharing it on the sports channels and pages of the universities in question. Finally, among the 563 questionnaires received, 384 questionnaires were analyzed. The tools of the present research were the adjusted questionnaire of the effectiveness of Kashmiri advertisements (2006) and the adjusted questionnaire of the attitude towards advertisements through sports Pyun (2006). The research tool was given to seven experts and professors of marketing to determine the validity of form and content. After reviewing the questionnaire by experts, the questions and the way of writing the questionnaire were changed in general, and both questionnaires were combined after checking and correcting the internal content. The advertising effectiveness questionnaire, which is derived from the AIDA model, has four components: Attention e.g. (to what extent have you seen the services and products advertised by famous athletes?), Interest e.g. (to what extent are you interested in the services and products advertised by famous athletes? ), Desire e.g. (to what extent are famous athletes' advertisements consistent with reality?) and Action e.g. (to what extent have the services and products advertised by famous athletes been able to influence the purchase confidence) that questions 1 to 7 are components of attention, questions 8 to 12 formed interest components, questions 13 to 17 formed desire components, and 18 to 23 formed action components. Also, questions 24 to 27 measured the variables of attitude towards advertising through sports. On the other hand, in order to determine the reliability of the research tool, Cronbach's alpha and composite reliability were used, and all values above 0.70 were obtained and approved (Mohsenin and Esfidani., 2013). Finally, the research tool with 27 items in the seven-point Likert scale from (very little to very much) was provided to the participants. The findings were analyzed in two parts; The first part investigated the demographic status of the participants by using descriptive statistics, and in the second part i.e. inferential statistics using the structural rate modeling approach of the data collected in two parts of the external model (measurement model) and The internal model (structural model) was analyzed. It is worth mentioning that SPSS22 and Smart PLS3 software were used in the analysis of findings.

Findings

In the descriptive statistics section and based on the information in the table below, it can be seen that male students have a significant share of 70.7% compared to female students. 65.8% of the statistical sample was in single status and the highest frequency related to the age of people in the age range of 18 to 25 years (approximately 41%) was assigned to them. Regarding the level of education, the highest frequency belonged to the undergraduate group with 36.7%. 62.7% of the participants were familiar with advertising through sports and the rest were somewhat familiar; More than 70% of the studied sample considered advertising through sports to be effective. Also, more than 80% of participants had seen advertisements of famous athletes at least three times. Also, in the final question of the demographic section, the participants were asked to choose the most effective type of advertising through sports among the options, which, respectively, are advertising in virtual space and the Internet and product endorsement by famous athletes (endorsement) as the most effective. They chose shares over other options.

In the inferential part of the research, the normality of the data was investigated, and the results are reported in Table No. 1.  

Table 1: Kolmogorov-Smirnov test results

Based on the results of the K.S test, it is concluded that considering that all the studied components have a significance level of less than 0.01, therefore, it does not have a normal distribution; therefore, in order to check the hypotheses of the research, it is necessary to use non-parametric statistical tests, and due to the nature of using the structural equation modeling approach in this research, its variance-oriented branch was the focus of the researcher in this research, because one of the characteristics of the variance-oriented is when the data is abnormal and in this situation, using the variance axis can be useful. Variance-based structural equation modeling consists of two main stages; 1) measurement model (external model); 2) Structural model (internal model) which is responsible for investigating the relationships between structures. According to Hair et al. (2014), indicators have been considered to evaluate each of the stages.

External model analysis

Factor loadings are calculated by calculating the correlation value of the indicators of a structure with that structure, which according to Hair et al According to the results obtained from Tables 2 and 3, we can confirm the factor loadings related to each question. Also, since the t-statistics of the questions that obtained appropriate factor loadings were not in the range of +1.96 (or in other words, they were obtained more than this value), their significance was also confirmed. The complete results can be seen in the following tables (tables 2 and 3).

Table 2- External model analysis evaluation

Table 3: factor loading coefficients of effectiveness components

In the section related to the evaluation of the reliability of the questionnaires (another requirement for verifying the external model), two statistical methods of Cronbach's alpha coefficient were used to evaluate the internal consistency of the questionnaires and the method of composite reliability or construct reliability of the questionnaires, and the results of both of these tests were considered appropriate (above the intended cut-off point of 0.7). In structural equation modeling, the extracted average variance index is used to evaluate convergent validity at the factor level. According to Hensler et al. (2016), the cut point of the desired index is estimated to be more than 0.5. Therefore, taking into account this and according to the results of the above table, the condition of convergent validity at the level of factors can be confirmed.

Table 4: The results of the divergent validity evaluation of hidden variables

In the evaluation of the differential validity of the research variables in order to ensure the validity of the test, the average variance extracted of each of the latent variables should be higher than the correlation of that variable with other latent variables Ritcher et al. (2016); therefore, the Fornell and Larcker criterion was used to examine this factor, and this condition was met in all hidden variables, and the discriminant validity, or in other words, the divergent validity of the appropriate research variables, was estimated.

After verifying the reliability of the external model, the next step is to evaluate the results of the structural model, which includes examining the predictive capabilities of the model and the relationships between the structures, which is done using a systematic approach in the form of steps based on the proposal of Hair et al. (2014). In order to evaluate the collinearity between the factors, the variance inflation index was used. According to Hair et al. (2014), the cut point of 5 tolerance thresholds is considered to estimate and detect the degree of collinearity between variables. Based on the results of the table below, the desired values were estimated.

Table 5: Evaluation of collinearity between variables

The next step is to evaluate the path coefficients and the significance of the relationships between the variables in the structural model, the information of which can be seen in Table 5.

Table 6 - The results of evaluating the direct and indirect effects of each variable

Based on the contents of the above table (number 6), it can be stated that all the hypotheses tested in the current research were statistically confirmed. For example, the effectiveness of advertisements resulting from the endorsement of famous athletes on students' attitudes towards advertisements through sports could play a role by obtaining a statistically significant and strong path coefficient (0.706), or in other words, more than 70% of the variance. The attitude of students towards advertising through sports has come from the effectiveness of endorsements by prominent personalities such as Mehdi Taremi. Or in another example, among the considered dimensions of the AIDA model for evaluating the effectiveness of advertisements, the attention-grabbing component has a greater share than other components. Other results are mentioned in detail in Table 5.

According to Hair et al. (2014), the coefficient of determination or in other words R2 is one of the main indicators regarding the evaluation of the structural model in the variance-based approach. The scope of decision-making regarding the evaluation of the coefficient of determination for each criterion variable has been considered in three intervals of 0.25, 0.5 and 0.75. This coefficient is calculated for each dependent variable; since in this research the dependent variable of advertising effectiveness consists of its 4 main components, the coefficient of determination for this variable has been obtained as 1; for another dependent variable - the attitude towards advertising through sports, this number reached 49% ; and according to the considered cutting points, it is possible to report the average coefficient of determination for the second variable.

Hensler et al. (2009) believe that the predictive power of the model, Q2 (Stone Geisser’s index) is another criterion for evaluating the structural model. This index helps researchers to determine the predictive power of the model, which is why they have considered 3 cut-off points of 0.02, 0.15 and 0.35 for model prediction. According to the findings (Table 7), it can be said that both attitude and effectiveness variables had high predictive power.

Table 7- structural model evaluation indices

Finally, after examining several indicators regarding the evaluation of each part of structural equation modeling (both the external model and the internal model), Hir et al. They introduced the variance axis, which is commonly referred to as the standardized root mean square residual (SRMR). The cut point in this index is considered to be less than 0.08. In this research, this cut-off point was also observed and its report is included in Table 8.

Table 8- Evaluation of the overall quality control of the structural equation model

Figure 2: Output of Smart PLS3 software in standard mode

Figure 3: Output of Smart PLS 3 software in meaningful mode

Discussion

Every organization can choose and use different types of advertising to reach its targeted market. Most companies/organizations tend to use the stars of the sports world to validate their brand, because athletes have more presence in the media than other famous people and because of their extraordinary capabilities and performance, create a lot of attraction for the customers and this can provide the necessary platform to influence the desired product as much as possible. The general purpose of this research was to estimate and evaluate the effectiveness of advertisements resulting from the endorsement of famous athletes on the attitude of students of  Sport sciences faculties in Tehran.

Based on the findings of the research in the descriptive statistics section, it can be stated that the statistical community of the research found advertising through sports to be effective and the same has been proven in the statistical analyzes of the inferential section. The participants stated that they are very familiar with sports personalities and for this reason, they had seen their advertisements to a significant extent in different ways. On the other hand, in the inferential statistics section, the results obtained around the existing assumptions all indicated that the 4 components of the AIDA model had a positive and significant effect on the effectiveness of advertisements resulting from the endorsement of prominent and famous sports figures, and at the same time, this effectiveness has led to the positive attitude of students.

In line with the interpretation and discussion of the results obtained from the analyzes carried out in this research, it can be stated that the use of prominent sports personalities in advertisements is a suitable tool for the effectiveness of advertisements and as a result influencing people's attitudes, especially the subject of study students of physical education faculties is in this research as an example. Various studies have sometimes confirmed this. As an example, we can refer to the research of Ivano et al. (2014), Boyle and Martins (2013), Ruhili et al. (2010), Peguraro et al. (2010), Keshmiri et al. (2016) and Eydi et al. (2015). In one of these researches Eydi et al. (2015), it has been stated that the more specialized the endorser athlete is showing good performance in competitions, the more effective the advertising will be, and the companies are more inclined to use an athlete in advertisements who has the mentioned capabilities.

Among the evaluation indicators of the effectiveness of advertisements in the dimensions of the AIDA model, the attention-grabbing component is; the component that was able to contribute the most in the effectiveness of advertisements from the perspective of students compared to other components in this research. Attracting people's attention is the first task of an advertising message. If the advertising message cannot do this task well, the effectiveness of the advertising message will be largely limited. The results of this research were consistent with previous researches including Keshmiri et al. (2016).

The component of awareness and interest are other steps of the AIDA model, and in this research, their effects on the effectiveness of advertising through sports were investigated and evaluated. In fact, the famous person and endorser must communicate with the customer and in order to provide a more effective advertisement, and he must learn about the way of thinking of the customers of the target market; therefore, advertising should be able to convince the consumer that the advertised product has the ability to meet the needs of consumers and customers and has played a significant role in creating awareness and interest. The review of the background of the research shows that the present research is not consistent with the researches of Keshmiri et al. (2016) and Daraei, (2014) because these studies did not declare the second step of the AIDA model in effective advertising. One of the reasons that can be stated for the ineffectiveness of these steps in the AIDA model is that both studies used environmental advertisements; As an example, in the research Keshmiri et al. (2016), which focused on advertising through sports, Irancell company took into account the advertising that Irancell company did in football events around the playing field, and according to the statistical community, this type of advertising cannot create awareness and interest in the mind of the audience; however, in this research, from the point of view of the target audience, the students were aware of the type of advertisement, and also the desired personality and also the interest in the famous person.

The component of creating a desire in the audience is one of the other steps of the AIDA model, and in this research, its effect on the effectiveness of advertising through sports was investigated and evaluated. The background of the research shows that one of the most difficult elements of the advertising message is to stimulate people to have or use the desired product. The advertiser must communicate with the customer and in order to do this correctly, he must know how the target customers think; therefore, the advertising message must be able to convince the customer that the product that it intends to introduce and supply can meet the customer's needs, which was shown by reviewing the existing literature, the results of this component with the research of Hosseini et al. (2009) is consistent. Also, from the point of view of the study of Dolat-Yaari & Poursoltani-Zarandi (2015), people have a positive attitude towards products or activities that benefit them, and this attitude affects their desire and motivation to use those products.

Finally, the last component of the AIDA model is the purchase action, which was investigated in this research. It should be kept in mind that in the fourth step of the IDA model, the customer's purchase is not considered, but in some cases, the emphasis on creating a positive effect for the purchase has been prioritized, because sometimes a special advertisement leads to a positive effect on the customer's purchase decision; but the customer is in a situation where he is not able to buy and implement it and therefore, when he finds the ability to buy, he buys the desired product (Dolat-Yaari & Poursoltani-Zarandi., 2015). Now, it can be stated that one of the successful solutions in advertising and creating a positive effect on the purchase of customers is the use of famous people, as Pardehan et al. (2016) state that the reputation and credibility of famous people and athletes are as It is that most people have a general knowledge of them, and this knowledge is enough to make a purchase in some cases, for example, in a country like Taiwan, 45% of company advertisements are done through authentication, because Its effectiveness on buying is more than other methods (Ojagh et al., 2017).

Conclusion

In general, based on the obtained results, it can be stated that advertising through famous athletes or endorsement and approval of products and services of companies and organizations by famous athletes has succeeded in affecting the attitude of students towards this type of advertisement in all four steps of the AIDA model, which includes attracting attention, creating awareness and interest, creating desire and stimulation and making a purchase. In this regard, this positive effect can be seen as the result of such things as the attractiveness and flexibility of sports, the beautiful and memorable performance of famous athletes, the controversial images of athletes, their personality type, ethics, and social behavior.

The current research was faced with limitations that can be considered to achieve better results and conduct research in the future. The study suggests that to identify the behavior of consumers, companies, and organizations should be aware of the attitude of their target customers and design strategies based on the attitude of their audience. Because creating a positive attitude in the audience leads to customer loyalty to the advertised product or service and vice versa. Therefore, to create a positive attitude in the audience, one should be careful in choosing the advertising method so that the opposite result is not achieved. Also, based on the findings of the study, researchers used indirect methods to evaluate the effectiveness of advertisements, but it is suggested that future researchers use direct methods such as increasing sales and using neuromarketing techniques to measure the effectiveness of advertisements.

Acknowledgments

The authors express their gratitude to all the students who collaborated in this study.

Conflicts of Interest

There is no conflict of interest.

Funding

The authors have used their personal financial resources for this research