need to write this part with more professional way and need example to it in what is the best theories.. A history of modern leadership theory: Three eras 
During the Industrial Revolution, leadership research became rigorous, using scientific methods to identify and measure leadership characteristics, with three eras contributing to our understanding.
The trait era late 1800 to mid 1940s
The belief that leaders are born rather than made dominated the late 19th and early 20th centuries, with historical context and social structures reinforcing this belief. This belief influenced leadership researchers, leading to the search for leadership traits. Studies on leadership traits assume certain traits distinguish leaders from followers. However, over 40 years of research show little evidence to support the idea that leaders are born. Some traits do matter, such as sociability, aggression, and livelyness, but the most relevant traits depend on the situation. Being social, aggressive, lively, original, and popular does not guarantee effective leadership in all situations. In the late 1930s and 1940s, researchers believed that traits were minimal in determining leadership effectiveness, viewing it as a group phenomenon. Recent studies in the 1960s and 1970s reinforced this, showing that intelligence and assertiveness are related but not enough to account for leadership effectiveness. Recent views on traits and individual characteristics have refined our understanding. Research on emotional intelligence and leader characteristics highlights the role of personality in effective leadership, but it's not the only or dominant factor.
The behavior era: mid 1940s to early 1970s
During World War II, researchers shifted from the trait approach to observable behaviors as a source of leader effectiveness. This shift was influenced by the dominance of behaviorist theories in the US and Great Britain, offering advantages such as easy observation, consistent measurement, and various teaching methods. Lewin's early work on democratic, autocratic, and laissez-faire leadership laid the foundation for the behavior approach to leadership. These styles were defined as consultative, decision-making, autocratic, and laissez-faire. However, research failed to determine the most effective style or situational factors. Each style had different effects on subordinates, with laissez-faire leading to disorganized groups, autocratic leading to submissive followers, and democratic leaders fostering relaxation and cohesiveness. 2,000 leadership traits, including task- and relationship-related behaviors, were found by the Ohio State Leadership Studies. Although the Leader Behavior Description Questionnaire (LBDQ) was created, it is still unknown how these traits relate to effective leadership. Although thoughtfulness and structuring actions are necessary for effective leadership, these requirements could not be applicable in cultures with differing leadership principles or beliefs.
The contingency era: early 1960s to present:
before the behavior approach failed to explain leadership effectiveness. Fred Fiedler's contingency model of leadership from the 1960s paved the way for the creation of more sophisticated models like the path-goal theory and the normative decision model. According to this paradigm, a leader's personality, management style, and behavior all rely on the circumstances they are in. It implies that there is no one best method to lead, that anyone can develop their leadership skills, and that both individual and contextual factors have an impact on a leader's performance.
Early theories:
Effective leaders utilize resources effectively and build strong relationships with followers to achieve goals, and early leadership theories addressed these challenges in various ways.
Fiedler’s contingency model 
Fred Fiedler's Contingency Model, the oldest and most researched contingency approach to leadership, suggests that effectiveness depends on the leader's style and situation, considering available resources.
Leader’s style: 
Fiedler's least-preferred coworker (LPC) scale determines a leader's style by analyzing their perceptions and descriptions of their least-preferred coworker. Low LPC scores indicate task-motivated leaders who focus on task accomplishment and self-esteem. High LPC scores, on the other hand, indicate relationship-motivated leaders who prioritize maintaining relationships and are easily bored with details. This difference is evident during times of crisis.

Hillary Clinton and Barack Obama are examples of leaders who demonstrate both styles. Clinton's role is to provide vision and inspiration while delegating agency management, while Obama's broader focus and less attention to detail indicate a relationship-oriented style. Brady W. Dougan, CEO of Credit Suisse Group, is detailed-oriented and task-motivated, while Marissa Peterson is task-motivated, focusing on clearly outlining the roles of her staff. Mort Meyerson, chairman and CEO of 2M Companies of Perot Systems, and Darlene Ryan, founder and CEO of PharmaFab, believe that leaders must be more in-tune with their people and customers to create an environment for successDarlene Ryan, a great listener and consensus builder, runs her company like a family, encouraging dissent and taking her time when facing tough decisions. Socio-independent individuals, who fall in the middle of the Leadership Personality Scale (LPC), may not actively seek leadership roles and may belong to task-motivated or relationship-motivated groups. Middle LPCs may be more effective than high or low LPCs in all situations. Despite validity issues, the LPC scale has strong support and has been translated for use in leadership research and training.
Situational control 
Leadership effectiveness relies on a match between a person and the situation. Fiedler's three factors to describe a leadership situation are the leader's relationship with followers, the task structure, and the leader's position power. The relationship between the leader and followers is crucial, as it provides a high degree of control. Task structure, on the other hand, refers to the clarity of a task, with highly structured tasks having clear goals and procedures. The leader's experience level moderates task structure, with more experience resulting in more structured tasks. The leader's position power, which refers to their formal power over subordinates, also influences their control. The combination of these elements determines the leader's situational control (Sit Con). Good leader-member relations, a highly structured task, and high position power provide high control, while poor leader-member relations, unstructured tasks, and little power lead to chaotic situations that do not provide a sense of control or ease of leadership.
Predictions of the contingency model: The Contingency Model suggests that a leader's effectiveness changes as the situation changes. Low-LPC, task-motivated leaders are effective in high- and low-situational control, while high-LPC, relationship-motivated leaders are effective in moderate-situational control. In high-control situations, task-motivated leaders feel at ease, while high-LPC leaders are likely to be bored and feel unneeded. In moderate-situational control, leaders lack cohesion or task structure, leading to ambiguous situations and task completion jeopardy. High-LPC leaders seek followers' participation and focus on resolving conflicts, using the group as a resource to accomplish tasks. Moderate control can be challenging for relationship-motivated leaders, as it can lead to autocracy and a lack of group support. Task-oriented leaders, like Nixon and Carter, are highly intelligent and focused on the task. However, as their power and popularity decreased, they became controlling, punitive, and ineffective. This pattern can be seen in former U.S. presidents like Nixon and Carter, who faced moderate control due to limited experience in foreign policy, poor relations with Congress, and a single-minded focus on human rights. Former presidents Ronald Reagan and Bill Clinton were high LPCs who focused on interpersonal relations and were popular with crowds. They enjoyed working with people and enjoyed moderate control. However, when a situation becomes chaotic and lacks group cohesion, low-LPC leaders take over and make autocratic decisions without concern for followers. This results in low performance and followers’ dissatisfaction. High-LPC leaders, on the other hand, face a nightmarish low Sit Con environment, making reconciliation efforts difficult and withdrawing to protect their self-esteem. Socio-independent leaders generally perform better in high-control situations, but more research is needed to predict and explain their performance. 
Evaluation and application: Over the past 40 years, the Contingency Model has been widely supported, despite criticisms about its validity, predictive value, and lack of research on middle-LPC leaders. Despite these concerns, the model remains a reliable and predictive leadership tool.
The Contingency Model suggests that leaders should understand their style and situation to predict effectiveness. They should focus on changing situations to match their style, develop strong relationships with followers, and receive training and experience to compensate for task ambiguity. However, the Normative Decision Model assumes leaders can change styles depending on the situation.
The Contingency Model suggests that leaders should understand their style and situation to predict effectiveness. They should focus on changing situations to match their style, develop strong relationships with followers, and receive training and experience to compensate for task ambiguity. However, the Normative Decision Model assumes leaders can change styles depending on the situation.
The Normative decision model:
Junki Yoshida, founder of Yoshida Group, demonstrates the importance of decision-making in his business. He actively participates in every aspect of the venture, allowing specialists to make decisions. The Normative Decision Model (NDM) suggests leaders should involve followers in decision-making, based on group dynamics and commitment. The model suggests leaders adjust their decision style based on importance and employee acceptance. Junki Yoshida, founder of Yoshida Group, demonstrates the importance of decision-making in his business. He actively participates in every aspect of the venture, allowing specialists to make decisions. The Normative Decision Model (NDM) suggests leaders should involve followers in decision-making, based on group dynamics and commitment. The model suggests leaders adjust their decision style based on importance and employee acceptance.
Leader’s decision style: 
The NDM identifies four decision methods for leaders: autocratic (A), consultation (C), group (G), and total delegation (D). Leaders must choose the appropriate style based on the situation and the problem involving a group or individual. Individual problems affect only one person, while group problems can affect a group or individuals. The distinction between individual and group problems is not always clear, as individual problems can affect others.
Contingency factor and predictions of the model: The Normative Decision Model considers various contingency factors such as the quality of the decision, the need for followers' acceptance, the leader's relevant information, the structure of the problem, the likelihood of followers' acceptance, employee agreement with organizational goals, cohesiveness, and the amount of information needed for decision-making. The Normative Decision Model, labeled "time efficient," suggests autocratic decision-making, when necessary, while a "time investment" version encourages participative decision-making. Autocratic decision-making is suitable when the leader has sufficient information, quality is not essential, or employees disagree. Consultative decision-making is suitable when employees demand participation, consultation helps gather information, and followers agree with organizational goals. Group-oriented decision-making is recommended when information is limited, quality is important, and employee commitment is crucial. Delegation assigns decisions to a single individual.
Evaluation and application: Research support the NDM in various settings, including historical evaluations and not-for-profit settings. The model's decision methods are defined and contingency factors are based on group dynamics and participative management. However, it may be too complex and subject to bias. The model suggests leaders must understand situations, use different decision methods, and pay attention to followers' needs.
Path-Goal theory:
The Path-Goal Theory suggests leaders clear subordinates' paths to achieve goals, fostering an exchange of guidance and support for productivity and satisfaction.
The Path-Goal Theory is based on the expectancy model of motivation, which suggests that individuals make rational choices based on their perception of the link between effort and performance. Leaders must consider the task and follower characteristics to determine which leadership behavior contributes to subordinate satisfaction, considering their needs and preferences.
The Path-Goal Theory, despite mixed empirical support, contributes to leadership understanding by focusing on guidance and support, employee satisfaction, and the leader's role as an obstacle remover. It also highlights the importance of challenge and autonomy in leadership.
Substitutes for leadership: Leader-follower relationships may not be necessary in certain situations, as work environments provide resources and support for subordinates to achieve goals independently.
The Substitutes for Leadership Model (SLM) suggests that various organizational, task, and employee characteristics can replace traditional leadership behaviors. Clear task information and support are often available, and the leader's lack of power or rigid culture can neutralize the effect. Open-book management, like Ricardo Semler's company, allows workers to set their own hours and receive detailed information. The Leader-Member Exchange (SLM) is a leadership model that addresses processes not considered by other models. It questions the need for leadership in certain situations and highlights the difficulty of being effective when many neutralizers are present. The model offers potential for application, as leaders may want to set up or remove leadership substitutes depending on the organization's culture, strategy, and goals. Autonomous and self-managed teams can also apply the SLM, using factors like training, professional orientation, and direct feedback as substitutes for leadership. The SLM suggests leadership training should emphasize effective leadership behaviors, setting up substitutes, and avoiding neutralizers, similar to Fiedler's Contingency Model. The dyadic relationship between leaders and followers remains a significant research area. 
Leaders member exchange: Leadership is often seen as a personal relationship between a leader and a subordinate, with each relationship varying in quality. The Leader-Member Exchange (LMX) model focuses on the unique, relationship-based exchange between leaders and followers, establishing roles based on social or economic exchange. The Leadership-Leadership Exchange (LMX) model suggests that a positive LMX leads to high performance, satisfaction, and low stress for in-group followers. This exchange is based on social rather than economic exchange and can impact job performance ratings, outputs, and organizational commitment. On the other hand, out-group followers face a different situation, with leaders often perceived as less motivated or competent, and their role limited to formal job descriptions. The LMX model suggests that the leader-follower relationship forms early and can be influenced by factors such as culture and performance expectations, Leaders select in-group members based on competence, performance, and commitment to the organization, avoiding favoritism and nepotism, and thus, formal human resource policies and procedures are essential. In certain cultures, fairness and equal opportunity are prioritized, with leaders hiring those they know or recommended by others. In others, loyalty to one's community is the primary concern, and leaders surround themselves with trusted family members. Organizational culture may also affect the quality of the team-oriented work environment. Interest in the Leadership Model Exchange (LMX) theory has grown, with recent extensions and testing in various cultures. However, researchers question the theory's adequacy, multiple measures, and methods used to test concepts. Factors influencing in-group versus out-group relationships need more attention, including personality similarity and perceived identity. More research is needed to identify factors affecting the development of LMX, assess the desirability of having two groups, and explore cultural factors affecting decision-making. The LMX model is appealing from a practitioner and application perspective, but further clarification is needed in areas such as gender similarity and organizational reality. Leaders often exclude highly competent employees from their in-group due to personal dislike or organizational politics, as leaders are subject to human error. In-groups can lead to abuse of power and membership in top management teams, as seen in the case of Michael Ovitz. Top-level executives aim to create a workable team with trusted and agreeable members, avoiding conflicts and differences. However, the Ovitz-Eisner case highlights the importance of being part of a team, as evidenced by Ovitz's firing from Disney. To counteract potential bias, companies should take time to connect with employees outside of their regular office routines. In-groups in leadership often consist of people with similar backgrounds and views, leading to problems in U.S. businesses and President George W. Bush's administration. To avoid these issues, leaders should select members based on competence, contribution, and performance. They should also evaluate criteria, assign tasks, set performance-related guidelines, avoid highly differentiated groups, and maintain fluid membership. Transactional-Transformational Leadership emphasizes the importance of leadership interactions and relationships.
Summary and conclusion:
The scientific approach to leadership, beginning during the industrial revolution, focused on leadership behaviors rather than simple traits, but struggled to accurately predict effective leaders. Modern leadership theories, such as Fiedler's Contingency Model and Normative Decision Model, focus on how leaders use resources and the relationship between leader and follower. The Contingency Model assumes leader's style is determined by internal traits, while the Normative Decision Model relies on learnable decision-making styles. Both models contribute to leadership application and training. 
Centralizing Purchasing:
As the western regional manager for a group of hospitals and clinics in eight western states, you have nearly 10 years of experience in purchasing with a major competitor. The health group oversees over 30 centers, with each operating independently. The purchasing managers from larger hospitals often compete for suppliers and fail to achieve economies of scale. To cut healthcare costs, the board of directors and president identified purchasing as a key area for savings. You are charged with centralizing purchasing and are expected to reduce costs by at least 15% within a year.