Victor Vroom

Valence is the importance placed upon the reward. Expectancy is the belief that efforts are linked to performance. Instrumentality is the belief that performance is related to rewards. For example a salesman's expectancy is his belief that more number of phone calls (effort) will result in higer sales (performance). His instrumentality is that higher sales (performance) will result in higher commissions (rewards). His valence is the importance attached to the commissions (rewards).

These three factors result in motivation. If any one of these factors doesn't exist then motivation vanishes. If the salesman doesn't believe greater effort leads to peformance then there is no motivation. Similarly, if commissions don't increase with sales then instrumentality disappears.