Adding correlation to the model accounts for the relationship between activities, which can result in increased variability in the model's outcomes. This will increase the standard deviation, which is a measure of the uncertainty in the model.
According to the PMBOK Guide, 6th edition, Chapter 11: Project Risk Management1, an effect of adding the correlation to the Monte Carlo schedule risk analysis model is that it increases the standard deviation of the model. This is because:
Correlation is the statistical relationship between two or more variables. In a schedule risk analysis, correlation can be used to model the dependency between the durations of different activities. For example, if two activities are positively correlated, it means that if one activity takes longer than expected, the other activity is also likely to take longer than expected. Conversely, if two activities are negatively correlated, it means that if one activity takes longer than expected, the other activity is likely to take shorter than expected.
A Monte Carlo schedule risk analysis is a simul-ation technique that uses random values for uncertain variables, such as activity durations, to generate possible outcomes for the project schedule. The simul-ation is repeated many times to produce a probability distribution of the project completion date and duration. The standard deviation is a measure of the variability or dispersion of the distribution. A higher standard deviation means that the distribution is more spread out and less predictable.
Adding correlation to the Monte Carlo schedule risk analysis model increases the standard deviation of the model because it introduces more variability and uncertainty to the simul-ation. Correlated activities can have a cumulative effect on the project schedule, either positively or negatively, depending on the direction and strength of the correlation. This can result in more extreme outcomes for the project completion date and duration, which increase the spread of the distribution and the standard deviation.
PMBOK Guide, 6th edition, Chapter 11: Project Risk Management1
Risk Management Professional (PMI-RMP)® Exam Cert Guide2
