Systems Thinking Mid Term Exam

The statement is true because inflow variables represent the rate at which something is added or flows into a stock variable, while outflow variables represent the rate at which something is subtracted or flows out of a stock variable. Since both inflow and outflow variables are directly related to the stock variable, it follows that they must have the same units. This ensures consistency and allows for accurate calculations and analysis of the system.

The desired level of aggregation of a model depends on the goal or function of the model. This means that the level of detail or complexity in a model should be determined by what the model is intended to achieve. Different goals or functions may require different levels of aggregation, whether it is a high-level overview or a more detailed analysis. Therefore, the statement is true.

The behavior of a stock, in terms of its change in level, can be described by a differential equation. This is because a differential equation is a mathematical equation that relates the rate of change of a variable to the variable itself. In the case of a stock, the rate of change of its level (e.g., price) can be influenced by various factors such as supply and demand, market trends, and economic conditions. Therefore, it is true that the behavior of a stock can be described by a differential equation.

Systems with explicit goals such as target inventory levels, temperature, number of labor, etc. are usually represented by Negative Feedback Loops. Negative feedback loops are used to maintain stability and bring the system back to its desired state by counteracting any deviations from the goal. In contrast, positive feedback loops amplify deviations and can lead to instability or exponential growth. Therefore, the given statement is false.

Runge-Kutta(4) is a numerical method that provides a more accurate approximation of the solution compared to Euler's method. It achieves this by using multiple intermediate steps to estimate the slope at different points within the interval. In a purely continuous model with oscillatory tendencies, the accuracy of the approximation becomes crucial. Runge-Kutta(4) is specifically designed to handle such scenarios and is known to provide better results than Euler's method. Therefore, it is reasonable to conclude that Runge-Kutta(4) will outperform Euler in this context, making the statement true.

In a simulation, a stock variable represents a quantity that accumulates over time, such as the total number of items in inventory. The value of a stock variable can only be changed by its flow variables, which are the rates at which items are added to or subtracted from the stock. For example, if the flow variable represents the rate of items being added to the inventory, then the stock variable will increase over time. Therefore, the statement that the value of a stock variable can only be changed by its flow variables is true.

The given answer, "Serial disaggregation," is the correct choice because it accurately describes the situation where a modeler shows the sequence of operations and inspections in the manufacturing process. Serial disaggregation refers to breaking down a process into individual steps or components, which is exactly what the modeler is doing by showing the sequence of operations and inspections. This allows for a detailed understanding of the manufacturing process and helps identify any potential issues or improvements.

The correct answer states that cognitive constraints and real-world decision-making need to be included in System Dynamics models if they have an important influence on the behavior. This means that when developing a System Dynamics model, it is important to consider the cognitive limitations and decision-making processes of individuals, as they can significantly impact the behavior of the system being modeled. By including these factors, the model can provide a more accurate representation of real-world dynamics and improve its predictive capabilities.

Pattern 1 (green) corresponds to the stock variable because it shows a similar behavior to the blue line in the graph on the top. The green pattern starts at a low point and gradually increases, indicating an inflow into the stock variable. This is followed by a decrease, representing an outflow from the stock variable. The overall trend of the green pattern matches the behavior of the blue line, making it the most suitable choice.

An inflow refers to the movement of something, such as money or resources, into a particular entity or system. Inflows are typically considered positive because they represent an increase or addition. Therefore, it is not possible for an inflow to be negative.

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In this system, the inflow of the stock variable is directly proportional to the stock level, while the outflow remains constant. This means that as the stock level increases, the inflow also increases proportionally, leading to exponential growth. The stock level will continue to grow at an increasing rate over time, without any oscillation or convergence to equilibrium. Therefore, the observed behavior in this system is exponential growth.

When the initial stock is reduced by half, it means that the new initial stock level is 200. The system then starts a goal-seeking behavior to reach the equilibrium stock level of 400. Since the inflow is constant at 360 units/day, the stock level will increase by 360 units every day. After 1.5 days, the stock level would have increased by 540 units (360 units/day * 1.5 days), resulting in a stock level of 740. However, since the system is trying to reach the equilibrium stock level of 400, it will decrease the stock level by 340 units (740 - 400), giving an approximate stock level of 350.

The modeler is expanding the model boundary by replacing clouds in an SFD model with stock and flows. This means that the model is being expanded to include more detailed components, such as stock and flows, instead of using clouds to represent them. This allows for a more granular and specific representation of the system being modeled.

Since the production rate is constant at 200 units/week and the raw material is ordered in lots of 250 units/week, there will be a surplus of 50 units/week. After 10 weeks, the total surplus would be 10 weeks * 50 units/week = 500 units. Adding this to the initial stock of 100 units, the stock level of raw material would be 600 units.

In a system dynamics model, an outflow represents the flow of a variable leaving a stock. In a causal loop diagram (CLD), a negative causal link indicates an inverse relationship between variables. Therefore, if an outflow leaving a stock variable corresponds to a negative causal link in a detailed CLD, it means that as the outflow increases, the stock variable decreases. This is consistent with the idea that outflows deplete stocks. Hence, the statement is true.

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Causal loop diagrams do not need to be corrected before simulation if they have crossed lines. Crossed lines in a causal loop diagram represent feedback loops, which are an important aspect of understanding complex systems. These crossed lines can be analyzed and interpreted during simulation to gain insights into the behavior of the system. Therefore, the statement is false.

A feedback loop can have negative polarity if it contains an odd number of negative link polarities. This means that if there is exactly one negative link polarity, the feedback loop will have positive polarity. Therefore, the statement that the polarity of a feedback loop is negative if it contains exactly 1 negative link polarity is incorrect.

The population of the country is currently 10 million and it has a constant doubling time of 25 years. This means that every 25 years, the population doubles in size. To reach 30 million, the population needs to double twice. Since it takes 25 years for one doubling, it will take 25 years for the first doubling and another 25 years for the second doubling. Therefore, it will take a total of 50 years for the population to reach 30 million. Since the options provided are in increments of 10 years, the closest option is "in about 40 years".

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If the stock variable only has one inflow, it means that the value of the stock variable is increasing over time. The inflow represents the rate at which the stock variable is being added to. Therefore, the corresponding behavior of the inflow is positive and increasing.

The units for the average experience of employees should be expressed as "week/person" because it represents the amount of experience gained per person. The units for the total experience of employees should be expressed as "week" because it represents the overall amount of experience accumulated by all employees in the company.

The unit of X is "day" because both inventory and shipment rate are measured in "item" and "item per day" respectively. A good name for X would be "average residence time" because it represents the average amount of time an item stays in inventory before being shipped out.

The equation p(t) = gp(t) - sp(t) represents the population growth or decline over time. The term gp(t) represents the annual number of births per person multiplied by the size of the population, which contributes to population growth. The term sp(t) represents the annual number of deaths per person multiplied by the size of the population, which contributes to population decline. Therefore, the equation p(t) = gp(t) - sp(t) takes into account both the birth rate and death rate to calculate the net change in population over time.

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A dynamic hypothesis is a hypothesis that changes over time. This means that the hypothesis is not fixed or static, but rather evolves and adapts as new information or data becomes available. It recognizes that circumstances and conditions can change, and therefore the hypothesis needs to be flexible and able to accommodate these changes. This is important in scientific research and problem-solving, as it allows for a more accurate and comprehensive understanding of complex systems and phenomena.

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Statement I is correct because a pipeline delay can indeed be approximated by a stock-flow structure in which many stock variables are connected. This is a common method used in System Dynamics to represent delays in the flow of resources or information.

However, Statement II is not correct. Euler is not necessarily the best numerical integration method for generating oscillations in a fully continuous System Dynamics model. There are other methods, such as Runge-Kutta or Adams-Bashforth, that may be more suitable for accurately capturing oscillatory behavior in the model.

A system with one negative and one positive feedback loop could demonstrate both exponential growth and S-shaped growth. Exponential growth occurs when the positive feedback loop dominates, leading to rapid and continuous growth. S-shaped growth, on the other hand, occurs when the negative feedback loop counteracts the positive feedback loop, resulting in a gradual increase that eventually levels off. Therefore, the correct answer is i and iii.

SD models do not aim to precisely reproduce real-world system behavior. Instead, they focus on capturing the essential dynamics and interactions of the system. SD models are simplifications and abstractions of reality, allowing for easier analysis and understanding of complex systems. They prioritize the representation of feedback loops, delays, and nonlinear relationships over precise reproduction. Therefore, the given answer, "False," is correct.

The given answer suggests that the stock variable converges to a level of 20 in the long-run, and it has a constant half-life of 3.5 time periods. This means that it takes approximately 3.5 time periods for the stock variable to decrease by half and reach a level of 10, and then another 3.5 time periods to decrease by half again and reach a level of 5, and so on. Therefore, the stock variable experiences a constant halving every 3.5 time periods.

A modeler who shows the different operations required to produce sub-assemblies of the final product is an example of parallel disaggregation. This means that the modeler is breaking down the assembly line into separate parallel processes, where each sub-assembly is produced independently. This allows for efficient and simultaneous production of multiple components, which can then be brought together to create the final product.

Link polarities describe the structure of the system. In qualitative casual loop diagrams, link polarities represent the direction of influence between variables. They indicate whether a change in one variable will cause an increase or decrease in another variable. By understanding the link polarities, one can determine the overall structure of the system and how variables interact with each other. The other statements in the options are not necessarily true for qualitative casual loop diagrams.

In this scenario, the modeler has found that the loading and unloading time at a certain machine is insignificant compared to the actual processing time. This indicates that the loading and unloading operations can be combined or aggregated together as a single operation. Therefore, the modeler should take the action of aggregation, which involves combining or grouping similar operations together to improve efficiency and reduce unnecessary steps. Serial disaggregation and parallel disaggregation would not be necessary in this case since the loading and unloading time is already negligible.

The correct answer suggests that in Model A, the Effect of Stock variable should be set to "Auxiliary - with Lookup" while in Model B, it should be set to "Auxiliary - Normal". This means that in Model A, the Effect of Stock variable is dependent on a lookup table, which allows for more flexibility and customization in determining the effect. In Model B, the Effect of Stock variable is not dependent on a lookup table and follows a normal auxiliary relationship.

The unit that needs to be used for the increase in production capacity is EV/month2. This unit represents the rate of change of production capacity with respect to time. It indicates the increase in production capacity per month squared, which aligns with the growth of expected demand for new EVs.

If there is a positive causal link from variable x to variable y and someone claims that y increases when x increases, but y also increases when x decreases, it implies that y is not directly influenced by x. This suggests that y must be a lookup variable, meaning its value is determined by looking up values from other variables or external sources, rather than being directly affected by x. Therefore, the statement "y must be a lookup variable" is true.

This model cannot generate damped oscillation because it does not contain any delays or time-dependent functions. Damped oscillation is a behavior where the stock variable fluctuates over time, gradually decreasing in amplitude until it reaches a stable equilibrium. Without any time-dependent functions, the stock variable in this model can only exhibit exponential growth, balancing growth, or constant behavior.

Based on the information provided in the graph, the number of parcels entering and leaving the warehouse is shown for each time interval. To determine the time interval with the smallest number of parcels inside the warehouse, we need to identify the interval where the difference between parcels entering and leaving is the lowest. From the graph, it can be observed that the difference is the lowest in the 10:00 - 10:05 am interval, indicating the smallest number of parcels inside the warehouse during that time. Therefore, the correct answer is that the number of parcels inside the warehouse was smallest at 10:00 - 10:05 am.