## 2 Literature Review
### Evm  
In 1967, the U.S. Federal Government agencies played a significant role in introducing EVM for the first time and using it in programs like large acquisition ones [9]. The U.S. Federal Government was also able to successfully take advantage of EVM in its widespread projects. Project managers could benefit from EVM the most as it keeps them informed of the early indications and procedures of a project's execution and alerts them to eventual corrective action if needed. A large number of research investigations have been devoted to this area, as shown in [5; 10], since EVM was introduced. Although EVM was employed to investigate time and cost both, the cost aspect has been the center of attention in most research studies. In a study, EVM was discussed from a price tag perspective [11]. In a later study, it is focused on predicting how long an activity takes by proposing a reliable methodology and using SPI [12] and a formula for estimating project time according to (EDAC) is proposed, i.e., Estimate of Duration at Completion. During the life cycle of a project, a point of reference is needed for EVM to measure the project's execution periodically. This fixed reference point is supplied by its project's baseline schedule. In order to obtain the time and costs of a project performance, the three key parameters of EVM must be compared to each other and the time and cost performance measures as below:  
* Budgeted Cost of Work scheduled (BCWS)
* Actual Costs Work Performed (ACWP)
* Budgeted Cost of Work Performed (BCWP)
* Earned schedule (ES)  
* Schedule Performance Index (SPI = BCWP)  
* Cost Performance Index (CPI = BCWP)
* EDAC = PD/SPI  
[PERSON] questioned the classic metrics of SV and SPI previously used since they failed to reliably forecast ending point of time of a project [10]. In this study, however, a time-based measure was designed to cope with false and unreliable behavior, which is dependent on similar principles to those of earned value method, but which renders the money-related metrics into a time aspect. He transformed the time performance metrics SV and SPI to SV(t) and SPI(t) and indicated that they showconsistent and reliable behavior throughout the project life cycle. EVM was also used for forecasting project duration, comparing the indicators of previous earned value performance, namely SV and SPI, with the recently developed indicators of earned schedule performance SV(t) and SPI(t) and later compared them with the results of time performance measured through the three methods of obtaining time performance in a project [13]. Their results indicated that a general schedule forecasting formula could apply to various project situations. Later, it was tried to evaluate EV-based methods by reviewing them as they are commonly used to forecast total duration of a project [14]. They delicately controlled the uncertainty level of project to conduct an extensive simulation survey in which they tried to account for the accuracy of forecasts as influenced by the network structure of project, and the reliable and accurate results obtained from the EV-based measures within time perspective. In their study, they investigated the capability of a newly designed method, and the schedule method earned, which hones the relationship between EV metrics and duration forecasts of the project. A new formula to calculate the (EAC) is presented [15], i.e., Estimate At Completion, of projects to hone EVMS. Their new method contains a formula which is composed of four variables: SPI, Scheduled Percent Complete by Duration (SPCD), Actual Percent Complete by Duration (APCD), and Sum of Durations to Due Time (SDDT). Studies measuring time performance of the project and/or the concept of earned schedule have been the focus of some publications issued in the popular and academic literature. Empirical data was manipulated by using statistical procedures involved in validation of time performance indices and stability investigations [10].