Porter and Lawler (1968) refined and extended
Vroom's (1964) expectancy model. They agree with Vroom that employee effort is
jointly determined by the valence that employees place on certain outcomes and
the degree to which people believe that their efforts will lead to the
attainment of these rewards. However, Porter and Lawler emphasize that effort
may not necessarily result in performance.
Furthermore, they contend that the
relationship between valences and expectancies, on the one hand, and effort or
motivation, on the other, is more complicated than Vroom's model suggests. 
The Porter-Lawler model holds that effort may not
necessarily result in job performance for two reasons. First, the individual
may not have the ability to accomplish the tasks that constitute his or her
job. In this case, even if the employee is highly motivated (i.e., expends a
lot of effort), performance may not be obtained. Second, the person may not
have a good understanding of the task to be performed (i.e., there may be a
lack of role clarity). High motivation will not result in job performance if the
employee does not have a clear grasp of the ways in which effort may be
appropriately directed.
In addition, this model highlights the point that
performance and satisfaction may not necessarily be related to each other.
Porter and Lawler define satisfaction as "the extent to which rewards
actually received meet or exceed the perceived equitable level of rewards".
The relationship between performance and satisfaction depends on several
factors. For example, in organizations, performance may not always result in
expected rewards. Employees are not likely to be satisfied in situations where
they are not given the amount of rewards to which they think they are entitled.
Furthermore, the Porter-Lawler model indicates
that the nature of the task has implications for the satisfaction-performance
linkage. That is, performance on a task may provide the employee with intrinsic
rewards, extrinsic rewards, or both. Moreover, the authors suggest that
intrinsic rewards can be more closely linked with good performance than
extrinsic rewards, because intrinsic rewards can result directly from task
performance. In contrast, extrinsic rewards are administered by the
organization. In other words, extrinsic rewards depend upon outside sources
both for recognition that performance has been attained and for the
administration of appropriate compensation. ;
In addition, the model holds that employees'
self-ratings of performance have a major impact on their beliefs about what
levels of rewards are equitable. The theory posits that rewards are not
evaluated in absolute terms; rather they are assessed subjectively. For
example, if employees believe that they have achieved a high level of
performance, they will think that they are entitled to greater rewards than
would be the case if they thought that their performance was at a low level.
Finally, Porter and Lawler suggest what may
happen after an employee performs. Specifically, the rewards that result from
a particular level of performance will interact with the employee's perception
of them to determine satisfaction. Thus, this model suggests that performance
leads to satisfaction, rather than the opposite. This was a significant
departure from the traditional thinking.