Name(s): Expectancy Theory of Motivation also known as Valence-Instrumentality- Expectancy Theory
Author: Victor H. Vroom developed the theory from his study on the motivation behind decision-making.
Classification: Cognitive or Need-to-Know Motivation Theories
Year: 1964, Porter and Lawler provided an extension to the model in 1968
The expectancy theory of motivation provides an explanation as to why an individual chooses to act out a specific behavior as opposed to another. This cognitive process evaluates the motivational force (MF) of the different behavioral options based on the individual's own perception of the probability of attaining his desired outcome. Thus, the motivational force can be summarized by the following equation:
MF = Expectancy X Instrumentality X ∑ (Valence(s))
Expectancy refers to the "effort-performance" relation. Thus, the perception of the individual is that the effort that he or she will put forward will actually result in the attainment of the "performance". This cognitive evaluation is heavily weighted by an individual's past experiences, personality, self-confidence and emotional state.
Instrumentality refers to the "performance-reward" relation. The individual evaluates the likelihood or probability that achieving the performance level will actually result in the attainment of the reward.
Valance is the value that the individual associates with the outcome (reward). A positive valance indicates that the individual has a preference for getting the reward as opposed to, vice-versa, a negative valance that is indicative that the individual, based on his perception evaluated that the reward doesn't fill a need or personal goal, thus he or she doesn't place any value towards its attainment.
As the Motivational Force (MF) is the multiplication of the expectancy by the instrumentality it is then by the valence that any of the perception having a value of zero or the individual's feeling that "it's not going to happen", will result in a motivational force of zero.
The expectancy theory of motivation seeks its roots from the University of Michigan where in 1957; Basil Georgopoulos, Gerald Mahoney, and Nyle Jones worked on a research program in organizational behavior. Their study focused on the conscious and rational aspects of employee motivation and the factors associated with levels of high or low productivity.
Their study evaluated the following three variables :
The hypothesis is:
"If a worker sees high productivity as a path leading to the attainment of one or more of his personal goals, he will tend to be a high producer. Conversely, if he sees low productivity as a path to the achievement of his goals he will tend to be a low producer." 
In 1964 Even though no formal theory of motivation emerged from this initiative, Vroom based the expectancy theory of motivation largely on the findings of earlier research. If fact, Vroom expanded the ideology to include the individual capacity to not only have a preference towards a certain goal, but to cognitively evaluate and rank them in order of preference. Thus, a particular reward can fulfill multiple outcomes, consequently adding to the sum of the valences.
Therefore even though individuals express high effort and high performance doesn't mean business success as people could be directing their efforts towards a doomed organizational goal. In addition, and contrary to popular belief, the expectancy theory of motivation provides an individual decision model.
The expectancy theory of motivation has been the target of many critics, Graen (1969), Lawler (1971), Lawler and Porter (1967 & 1968), since it was originally presented by Vroom in 1964. These critics are far more an extension to the original concepts as opposed to a deviation from them. Actually Mr. Vroom admitted himself that the expectancy theory of motivation should be updated with new research findings. 
One of the major criticisms of the expectancy theory of motivation decision model was its simplicity. In the sense that it doesn't explain the different levels of efforts acted out by an individual. There is also the assumption that a reward will entice an employee to expand greater efforts in order to obtain the reward, but neglect the fact that the reward in question could have a negative effect for the individual. For example a pay increase might push him or her into a higher tax bracket.
The effectiveness of the expectancy theory of motivation decision model from a managerial perspective relies on the manager to make assumptions on the motivational force of the reward for the employee (s). Thus, the uses of the "rewards" need to obey to "The Law of Effect" where:
The expectancy theory of motivation has prevailed as an acceptably rational explanation for an individual decision-making model. It's without question that the theory which is a predictive value may enable managers to increase the likelihood of an individual acting out the desired behavior. However, the implementation of the theory in an organizational context isn't an easy task! Many sub-cognitive processes are involved in the overall decision that finding the balance between the individual's "reward" and the cost to be borne by the organization becomes a tedious task. In addition, each trial changes the equation, in the sense that the individual will use that new experience to alter his or her perception of the future probability of attaining the desired outcome.
The evolution of cognitive neuroscience techniques, brain scans being among the major ones, could expand the understanding of the cognitive processes involved in our decision-making process. More specifically they perhaps could shed some light on the "evaluative" as well as the "progressive" nature of motivational forces that drive our behaviors.
Sharing your motivating thoughts or your motivational tips will benefit every leader. Motivation is what give them the energy to constantly seek to improve their self-motivation as well as motivating other. Consequently increase our leadership influence.